Colours in the Environment: PART 1: Sky, Clouds, Sunsets, Twilights, NLCs; Some Fascinating Physics in Play; Interaction of Light with Matter

 Blog Contents and Who am I?

Click on a slide to view its fulll page image

Our lives will be less interesting without the display of wonderful ever-changing panoramic colours in the environment - the sky is blue during the day but at sunrise and sunset it turns red/orange; clouds are white and grey but take some striking hues; oceans are blue or deep green, and may even look red sometimes.  Ice is considered white - then why a glacier, a big chunk of ice, deep blue?  We have all experienced the surprise and delight when the majestic rainbow  suddenly appears -- a double rainbow is just awesome.  

Some fascinating physics is in play to enrich our lives with these (and many other) wonderful displays.  The physics is about the way sunlight interacts with particulate matter and gas molecules (mostly oxygen and nitrogen) in the atmosphere. Light interaction with water molecules in liquid (swimming pools/ocean) and solid state (glaciers) is completely different. The physics of rainbows is different still and has already been discussed in detail here. 

In the atmosphere, how the particles and gas molecules scatter sunlight depends on their size, number density and on the colour (wavelength) of light.  In liquid water and ice, the internal structure of water molecules determines how longer wavelengths (yellow and red colours) are removed more efficiently than shorter 'blue' wavelengths.  

In this article, I shall delve into the science of colours in the environment with the aim to convey a greater understanding of how light and matter interact.  In Part 1, I shall discuss the colours of the sky and clouds, and deal with the case of bulk water and ice in Part 2.

You might also like________________________________________

Physics of Rainbows; Moonbows; Fogbows - All You Need to Know - an Outreach Feature for the Community

Colour of Objects: Visible Light Spectrum; Primary Colours; Optical Illusions; How Do Colours Work?

_______________________________________________________________________

To maintain continuity of presentation, some background information will be included in the Appendix.

PART 1:   Why is the sky blue during the day but at sunset it turns red?

Slide 1

As you would expect, the spectrum of light (wavelengths present) during the day and at sunset/sunrise are very different.  The detailed spectra depend on atmospheric conditions like pollution, clouds etc., but the following slide is a good example and shows the main features:

Slide 2

The way light is scattered by particles in the Earth's atmosphere is responsible for the changes in the spectrum from day to sunset.  First, we take a look at the atmospheric constituents and then study the scattering of light by these particles.  Without the atmosphere, there will be no scattering of light and the Sun will always look white and the sky pitch dark. The photograph of the Sun and surrounding sky taken from space where there are no atmospheric particles (eg from the space station or the Moon) is shown in the following slide:

Slide 3     

The Atmosphere and its Constituents:  The atmosphere extends to an altitude of 100+ km and is divided into several layers - the most important are shown in slide 4. Troposphere is the most relevant for our discussion - it contains ~80% of the atmosphere by mass and ~99% of all water and particulate matter.  Clouds and weather happen in the troposphere.  Particle size determines how light is scattered by them.  This is summarized in slides 4 & 5.

Slide 4

Slide 5

Scattering of Light by Atmospheric Particles:

I have added a brief description of the physics of how light interacts with matter particles in the Appendix.

It is convenient to identify three scattering regimes (see last column of slide 5). The important number is the scattering parameter (x = 2 𝜋 r/𝜆) which tells us how big or small a particle of radius r is relative to the wavelength of visible light (𝜆); in our case, 𝜆 covers the range from 450 nm to 700 nm. For gas molecules (oxygen and nitrogen),  the radius is equal to 0.15 nm and  x is of the order of 0.0015 - much smaller than 1; and only Rayleigh scattering needs to be considered.  Mie scattering is important when x is of the order of 0.1 to 1.0.  For larger values of x, the scattering is called geometrical (aka optical).  Slides 6,7, 8 & 9 explain the situation. Only Rayleigh scattering is strongly wavelength sensitive (1/𝜆⁴ dependence) with blue light at 450 nm scattered almost five times more strongly than red light at 650 nm.  

Slide 6

Slide 7

Slide 8

Slide 9

The probability of light scattering from a particle increases as particle size gets bigger.  Very roughly, Mie scattering can be >10000 stronger than Rayleigh scattering (see slide A2) while optical scattering increases as the square of the particle radius (click here for quantitative details).  However, note that the number density of molecules that cause Rayleigh scattering is extremely large (see slide 5), and at higher altitudes  the contribution of Rayleigh scattering is the most important.  Even with the large number of gas molecules (of the order of 10²⁵ per m³), a light photon only experiences a single scattering event in passing through the Earth's atmosphere. 

The density of aerosols at high altitudes is very low, and only at altitudes less than 5 km Mie scattering by aerosols in the troposphere becomes significant.  At even lower altitudes, geometric scattering by clouds and pollution particulates may dominate - depending on the weather and contaminants generated by natural and/or human activities.   

Another interesting feature to note is the angular distribution of scattered light - Rayleigh scattering shows a modest forward and backward peaking with only half as much light scattered along perpendicular directions (scattering is peanut shaped - see slide 6). Mie scattering is forward peaking and becomes more so for bigger particles.  In optical scattering, due to multiple scattering effects, all wavelengths are scattered more or less uniformly in all directions; for bigger particles like raindrops or ice crystals, light can enter into the particle body and produce phenomena like the rainbows.   

The Sky is Blue During the Day: We are now ready to not only explain why the sky appears blue during the day but also to analyse the varying shades of blue when one scans the sky.  Basically, the variation in blueness of the sky is due to the mixing of Rayleigh scattering (RC) with different amounts of Mie and optical scattering (MO).  

Slide 10

Slide 10 is best viewed by clicking on it for a full page image.  The blueness of the sky at various points is well reproduced by mixing different amounts of MO to RC. In fact a full spectrum of blue hues from quite sharp to very soft may be obtained this way.  This is explained in slide 11.

Slide 11

It is not advised to look directly at the Sun which is more or less white with some yellowish hue. The sky near the Sun is less blue than the sky at angles further away from it. In regions close to the Sun, some blue light is indeed scattered out from the white sunlight by Rayleigh scattering but the overall light is still nearly white. Slide 12 explains the colour of the sky near the Sun.  

Remember that when you look at any part of the sky  you are observing its features in a very small angular range (much less than a degree) and the light from the Sun can only reach you if it is scattered out of sunrays by some object.   For example, the line of sight along the direction YA shows how the scattering by aerosols (Mie and Optical scattering), redirects white light from sunrays towards the observer and dilutes the blueness of the sky due to RC.  In the absence of any atmosphere, the Sun will appear as a white disc surrounded by black sky (Slide 3). 

Slide 12

The contribution from Mie scattering decreases as one looks away from the Sun and the sky becomes progressively deeper blue.  Along the horizon away from the Sun, the path length of sunrays in the lower atmosphere is rather large and while Mie scattering contributes little, optical scattering by larger aerosol particles becomes important and the sky colour has much more whiteness (see slide 1).

An interesting observation of the scattering of light from the atmosphere is made by high flying U2 planes. They fly at about 20 km altitude - above the troposphere - and can receive Rayleigh scattered blue light from gas molecules in the atmosphere.  Also note that the outer space looks pitch dark - there are no molecules or aerosols there to scatter sunlight back towards the Earth.

Slide 12a

The Colour of Clouds: During the day, clouds are largely white with shades of grey. The base of large clouds is generally dark (slide 10 and 13).

Clouds consist of water droplets and/or ice crystals.  Majority of the clouds are in the troposphere  and come in many shapes and sizes.  Clouds form by water molecules condensing on an aerosol particle (dust, sea-salt, soot etc. typically of size around 0.0002 mm or 0.2 micron).  Cloud droplets may vary in size from about 20 to 3000 microns.  Low clouds are found at altitudes below about 2 km, but high clouds can form at heights from 5 to 20 km or even higher - they are generally thin and are not associated with weather. 

Cloud thickness is an important parameter for our discussion; it varies enormously depending on the cloud type. Clouds can be quite thin - a few hundred meters (e.g. cirrus) to > 10 km in thickness (e.g. cumulonimbus).  

Sunlight is efficiently reflected by water droplets and ice crystals - more or less isotropically (equally in all directions) - very little light is actually absorbed by the clouds (slide 13).  

Slide 13

Generally, light does not penetrate more than one km thickness of the cloud and the base of a thick cloud appears dark (slides 1, 10, 14).  

Slide 14

Why Are Sunsets Red?

Sunsets and rainbows are widely photographed and viewed natural phenomena.  We are fascinated by the panoramic display of the large variety of colours at sunsets - these displays do not last for a long time and that only adds to their fascination.  If clouds are present, sunsets illuminate the sky with colours ranging from yellow, orange, pink to deep red.  Why?

(Our discussion applies equally to sunrise; sunsets tend to be more impressive due to larger concentration of particulate matter in the lower layers of the atmosphere in the evenings)

The transition from day to night is more complex and the details are not generally appreciated.  Starting with sunset, several different stages happen - these have their origin in disparate physical processes.  I have briefly discussed these in the appendix. 

Slide 15

Slide 16

In simple terms, the colours seen at sunset represent the relative absence of blue and green in the sunlight that we receive (slides 2 and A1).  Clouds are highly reflective and the the reddish hue at sunset is due to bottom of the clouds scattering light incident on them - light that is rich in longer wavelengths.  

There are two important reasons why light at sunset is blue deficient.  At sunset, 

(1) Light travels in the atmosphere through a much greater distance than during the day and a light photon is likely to be scattered multiple times. 

(2) Light passes through the lowest part of the atmosphere where large size aerosols are found. 

Path Length at Sunset in Atmosphere:

Slide A3 (appendix) gives a simple derivation of the path length D through the troposphere (of height 10 km) for various elevations of the Sun.  D increases to over 500 km when the Sun elevation is 1⁰. At this elevation, to reach the observer, light will travel 2500 km in the stratosphere. Actually, at sunset refraction in the atmospheric layers increases the path length even more.  For such long path lengths, light scatters many times from gas molecules (Rayleigh scattering);  on  each scattering, five times more blue light is scattered out of the sunrays (and hence lost) than red light.  By the time light reaches the observer, it is highly deficient in blue and green, and consists mainly of longer reddish wavelengths. 

Secondly, the lower atmosphere consists of aerosols particles of larger sizes which efficiently scatter light in all directions - this further reduces the amount of blue light (as well as longer wavelengths).  The result is an overall attenuation of light in the lower atmosphere, making sunsets less bright. The light, rich in yellow to red colours, reflects from the bottom of surrounding clouds and makes them appear bright yellow, orange, pink or red. 

Another interesting feature to note (slide 15) is that the sky at larger elevations of greater than a few degrees still appears blue - although not as bright as it is during the day.  This happens because light from the setting Sun still reaches upper layers of troposphere and stratosphere where it is Rayleigh scattered by gas molecules towards the observer - scattered light received by the observer is rich in blue colour. Density of molecules in the upper atmosphere is not large and the overall the sky is not as bright as it is during the day.

Refraction of Light in the Earth's atmosphere - Total Lunar Eclipse:

At sunset, sunrays pass close to the Earth's surface where the pressure and hence the gas density changes rapidly with altitude.  Sharp density gradients cause light path to bend (see slide 17) and the Sun to appear higher in the sky than it really is.  The change in elevation depends on atmospheric conditions but is of the order of 0.5 degrees.  Light from the lower parts of the sun's disc is refracted by a greater amount and the apparent position of the bottom of the Sun is shifted more than the top of the Sun - giving the Sun a squeezed appearance (oblate disc shape).  

Slide 17

Colour of the Moon at Total Lunar Eclipse: An interesting consequence of refraction of light in the Earth's atmosphere is during a total lunar eclipse when the Moon is seen to acquire a reddish brown colour at the time of total occlusion. This happens when the Earth is directly in the path between the Sun and the Moon and casts a shadow big enough to totally block any sunlight from reaching the Moon.  Normally this would imply that the Moon disappears from our view.  However, rays from the Sun still pass through the Earth's atmosphere and refraction causes their paths to bend in such a way that Moon is illuminated by the refracted sunlight.  Because the light has passed through a big length of Earth's atmosphere, Rayleigh scattering causes blue (shorter) wavelengths to preferentially scatter out of the sunrays leaving the transmitted light rich in longer red wavelengths.  The intensity of such light that reaches the Moon and is then reflected back to the observers on Earth is rather weak and the Moon, at the time of total eclipse appears muddy red.

Slide 18

Earth's Shadow, Sunset, Twilight Zones and Dusk:     

It is not widely appreciated that even though the troposphere has about 80% of the atmosphere mass, the stratosphere and mesosphere (slide 4) hold appreciable amount of gas molecules.  Besides the ozone layer at about 25 km altitude, there are aerosol particles which may stay in the stratosphere for long periods of time - such aerosols reach the stratosphere from volcanic eruptions, high flying aeroplanes etc.  Dust from meteorites in the mesosphere and beyond consists of very fine nanometer sized particles that act as water condensation centres and form clouds.  

During the day, scattered light from the constituents of the stratosphere and mesosphere is indeed present but is masked completely by the much more intense scattered light by gas molecules in the troposphere.  In the evening around sunset through twilight zones (our discussion is equally valid for sunrise) Earth's shadow blocks sun rays from illuminating the troposphere, but sunlight can still reach higher altitudes of the atmosphere (slides 19, 20 and 21).

Slide 19

Slide 20

Slide 21

For a description of the various form of twilights, see slides A5 and A6 in the appendix.

Scattered light from the higher layers of the atmosphere may consist of both Rayleigh scattered light from gas molecules and also light reflected by clouds and small number of larger aerosol particles. Path lengths are particularly long and the twilights are strongly reddened.  Twilight colours are particularly striking during the civil twilight period (soon after the Sun sinks below the horizon). Slide 22 shows some examples; Click here for more lovely images of twilights.

 Slide 22

Noctilucent (NLC) Clouds:  This article would not be complete without a discussion of NLC clouds - also known as night-shining clouds.  NLCs are located at altitudes of around 80 km in the upper layers of the mesosphere.  They are too faint to be seen during the day.  When the Sun is below about -10⁰ and the lower layers of the atmosphere are dark in the Earth's shadow, light from NLCs present a wonderful enchanting display in the mid latitudes during the summer months.  Polar regions never get dark enough for NLC to be visible.    

 Slide 23

NLCs are made of tiny ice crystals that are formed by water vapour freezing on extremely cold, < -100⁰C, meteoric smoke - the microparticles (nanometer sized) created when meteors burn up in the upper atmosphere.  In 2007, NASA launched their AIMS satellite to study noctilucent clouds and the complex science of the upper atmosphere - AIMS data has provided much information and understanding about NLCs. AIMS has observed that noctlucent clouds have been steadily increasing over the past decade - this is possibly related to the increase in water vapour (in summer months, relatively wet air circulating up from the lower atmosphere brings extra water vapour to the mesosphere) and decreasing upper atmosphere temperatures that are a side effect of the recent troposphere warming (climate change).  NLCs are also being sighted at more southerly latitudes - as far south as London and Paris due to cooling of the mesosphere.  See some remarkable pictures of NLCs on the facebook NLC group page.

Postscript:  

1.  For the sake of simplifying the account of interaction of light with matter, I have ignored the polarization of scattered light.  Polarization of Rayleigh scattered light brings in many interesting observations but I have felt that such a discussion would obscure the attractiveness of the subject at the community outreach level. 

2.  The original blog has been divided into two parts - the second part deals with the colour of water and ice.  Again, as we have seen in this part 1, things that happen in the environment are never simple with many processes contributing at the same time.  It is, however, possible to peel off the extras and concentrate on the most important science - for the sky it is the scattering process.  For the interaction of light with water, absorption of longer wavelengths by water molecules is the determining factor. Water (H2O) is special where its  play an important part in determining its interaction with light photons and the physics is really exciting.  This is the subject of Part 2.   

Post Postscript:  Also of interest: 

Rare 'mother-of-pearl' cloud spotted in Scotland

 https://www.bbc.co.uk/news/uk-scotland-64450253 

APPENDIX

Slide A1

Scattering of Light by Particles: Light is an electromagnetic (EM) wave - a manifestation of oscillating electric and magnetic fields.  All matter consists of atoms which have a cloud of negative electrons located around a massive (about 2000 times heavier than the total mass of the electrons) positively charged nucleus.  The electric field of the incident light causes the electrons to oscillate - an oscillating dipole forms which itself radiates EM waves.  Waves from the oscillating dipole combine with incident light to cause modification in the way the incident light propagates - its direction may be affected - this is the process of scattering.  In a single scattering event, most of the light remains unaffected and only a very small portion suffers a change of direction.

The detailed theory is the Mie theory.  In the limits of very small particle size, Mie theory may be simplified to give Rayleigh Approximation.  Similarly, for large particles, Mie theory converges to the simpler Optical scattering.  This is explained in slide A2.

Slide A2 

Slide A3

Slide A4

See also 

Transition from Day to Night:

Slide A5 shows the stages as the sun sets below the horizon.

Slide A5

Slide A6

Copyright Issues:  Most figures have been drawn by myself and may be used provided the blog address is acknowledged.  There are 3 slides (slides 1, 12a and 16) where I have not been able to locate the copyright holder - if they contact me then I shall be delighted to add acknowledgement to their work.  

Artificial Light at Night is a Rapidly Growing Problem with Serious Health & Ecological Implications; Solutions are Relatively Simple: A Public Information Feature

  Blog Contents and Who am I?

"Air, noise and water pollution have been high priority policy issues for decade, but light pollution remains scientifically, culturally and institutionally in the the dark"   ...The Atlantic 2019

Light and dark are concepts deeply embedded in our cultural and genetic heritage.  For a lot of people, it is hard to comprehend that light can have negative consequences.  The past 150 years have witnessed an explosion of artificial lighting (man made illumination) that has seriously disturbed the predictable natural rhythm of day and night.  The evolution of living organisms has been  controlled by the natural cycle of light and dark. Since the introduction of electric light bulbs, anthropogenic light has disrupted this status quo with many adverse consequences, not only for humans but for animals and plants too.

The two main problems with artificial light are the excessive amount of light illumination and its colour spectrum.  Urban centres are the regions where light pollution is the most serious. Urbanisation is a megatrend with a projected 70% of global population living in cities by 2050, and one expects that adverse effects of light pollution will continue to increase. 

(Click on a slide to view it full page image) 

Bright lights from cities are clearly evident when viewed from space.  Some of the city light is scattered back to the Earth by dust and gas molecules in the atmosphere, and gives the sky a diffuse brightness - what we call the skyglow.  Skyglow can make the city sky more than a hundred times brighter than the sky in rural areas far away from man-made lights.  Skyglow is so prevalent that several billion people (80% in the USA and >60% in Europe) cannot see the Milky Way (Click here for some good pictures). Astronomers measure skyglow from levels 1 to 9 on the Bortle scale; watch this 3 minute video to see how stars and the Milky Way are disappearing under the impact of artificial lights. 

It is also a serious threat to nocturnal wildlife. On a cloudy day (when there are more particles in the atmosphere), much more light is scattered back to the Earth and night skies over cities can be more than 20 times brighter than on a cloudless night (more details here). 

 

Other forms of light pollution are known as clutter, trespass and glare. The terms are self explanatory - there are many good articles on these topics and I refer you to 1, 2, 3 and 4.

 

In this publication, I shall look at the science of light pollution with particular emphasis on the effect of recently introduced light sources (compact fluorescent lights CFL, and light emitting diodes LED) on chrono-biology.  The next slide lists some of the impacts of light pollution:  

Light Pollution and Human Health:  For millions of years, human physiology has been governed by our internal biological clock or circadian rhythm (CR).  CR follows a rough 24-hour cycle - it not only controls your sleep/wake pattern but many body functions rise and fall at set times of the day based on your circadian rhythm, see also. For the importance of sleep, I refer you to 1 and 2.

Circadian Rhythm and your sleep/wake pattern may be affected by the amount of ambient light (jet lag is a good example where the time shift after a long plane journey may expose you to a diffrent day/night cycle) and by its spectral properties (intensity distribution with wavelength or colour).  Night shift workers are particularly vulnerable and suffer many serious health problems directly attributable to the unnatural ways that they are exposed to artificial light.  New artificial light bulbs introduced over the past 50 years are economical to run but have a spectral signature that is relatively rich in short wavelengths (blue light).  Exposure at night to blue light of wavelength around 480 nm has been shown to be particularly detrimental to maintaining a natural circardian rhythm.  

 

Understanding Artificial Light (AL) :  Artificial light (man made light) has made dark periods of evenings and nights  accessible for human activities that, otherwise, would have been impossible. Some aspects of artificial lighting are at odds with our evolutionary biology, and there is growing unease about the way we use/abuse AL. Our circadian rhythm evolved with natural light and requires exposure to bright light in the mornings and longer wavelengths of light (yellow-red) during evenings (see slide).  

 

The old style incandescent light bulbs (ILB) emit light by heating a tungsten filament to high temperatures around 3000C.  The light spectrum, as it happened, closely matches  the spectrum of sunlight at dusk.  While overillumination (use of excessive unneeded light outdoors) was a problem, artificial light from ILBs did not cause serious disruption of circadian rhythm. 

 

Around 1950, significant changes happened in the way artificial light is generated.  ILBs were inefficient (~10%) with  90% of the electrical energy wasted as heat.  Far more efficient (~ 50%) compact fluorecscent light (CFL) arrived first, with super efficient (~85-90%) light emitting diodes (LED) have now completely replaced ILBs.  Unfortunately, increased efficiency made light inexpensive and Jevons paradox kicked in.  AL is now used excessively and needlessly with overillumination becoming a serious problem causing skyglow, clutter, glare and tresspass. Human societies now live in contradiction with their biological heritage - they are indoors during the day with less exposure to bright lights, and then they switch on bright lights of the wrong spectral makeup during the evenings and night - exactly opposite to how we evolved.    

The hue of a light source is rated according to its CCT value.  This is explained in the next slide:

The spectral profiles of LED and CFL bulbs are shown in the next two slides:

In the past, sodium discharge lamps were used for outdoor light illumination.  Sodium lamps are being replaced by the much more efficient LEDs.  While the CCT of sodium lamps is around 2200K and they appear yellow, LEDs tend to have a much higher CCT value and there is a lot more blue tinge in their light.

The following slide shows the remarkable difference in the Milan city lights after HPS lamps were replaced by LED lights (as seen from space):

Milan is typical of most big cities.  The slide demonstrates that much of the outdoor lights that are meant to illuminate the roads and infrastructure is sent upwards and therefore wasted.  This is a good example of unplanned, careless and wasteful use of energy that not only adds to costs but also sends large amounts of greenhouse gases into the atmosphere contributing to global warming. 

Artificial light consumes 19% of global electricity production and emits 1.9 gigatonnes of CO2.  The worldwide use of artifical light is increasing by about 6% per year.

Production of Many Hormones is Affected by Light: Several hormones show daily oscillations (circadian rhythm).  Melatonin (controls sleep), cortisol (activity and stress), gonadal hormones progesterone & testosterone (sex), thyroid and growth hormones are known to have circadian rhythm.  Nutrient sensitive hormones like insulin, leptin, ghrelin also oscillate on a circadian basis, and their release is at least partly regulated by light-dark cycles.

In order to emphasize the importance of maintaining a good circadian rhythm by the provision of proper light-dark cycle and correct spectral quality, I shall look at the particular case of melatonin production. Melatonin is called 'the sleep hormone'; its production increases as dark hours set in.  Blue light has been shown to suppress melatotin synthesis:

Melatonin synthesis in the pineal gland requires availability of serotonin that in turn is made from the essential amino acid tryptophan.  Tryptophan is present in high protein foods like egg white, cod, cheese, sunflower seeds etc. and efficient synthesis of serotonin happens in the presence of bright lights.  In fact, there is good evidence that mood seasonality  - seasonal affective disorder (SAD) - is due to low availability of serotonin.   Being in bright sunlight during the day for an hour helps serotonin production which in turn helps in melatonin synthesis.  It seems that for ensuring good sleep, it is also important to spend some wake time in bright sunlight. 

The above discussion establishes that for efficient melatonin production blue light exposure must be minimised after evening sets in. This should ensure proper sleep (lasting 7 to 8 hours) that is essential for maintaining circadian rhythm.  Why sleep is important and how our health is affected by poor quality and/or insufficient sleep has been comprehensively covered by my publications (1, 2) and I refer you to them for details. 

Impact of Light Pollution on Animals and Plants:  Animals and plants belong to the Earth's ecosystem as much as humans do - we all have evolved together over millions of years under the daily rthythm of light and dark. Compared with other environmental features like temperature, atmospheric constituents, climate etc., light and dark cycles have been extremely stable, and biological systems are unlikely to have developed the ability to easily/rapidly adopt to sudden changes in the level of light at night.  

Artificial light at night (ALAN) has possibly disrupted the rhythm of life far more for animals and plants than it has affected humans.  ALAN pervades large geographic areas - for example, the sky glow from cities may be visible from hundreds of kilometres away.   With the exception of more general loss of view of the Milky Way and stars, ALAN only affetcs humans living in cities.  Humans  can also control their exposure to ALAN, but nocturnal animals have no such options available.  Nocturnal animals have used light from the Moon and stars (brightness ~0.1 lux) for activities like migration, searching for food, habitats and mates. Excessive brightness of ALAN can seriously disrupt animal physiology, biological timings, behaviour and welfare - it has the potential to disrupt entire ecosystems. Many animals (fish, insects, turtles) have high sensitivity to blue light; the recent widespread use of LED lighting, that has a high content of short wavelength blue colour, have further accentuated the adverse effects of ALAN.  

Over the past 25 years, scientists have started studying how ALAN affects animals (both terrestrial & aquatic) and plants, but much work remains to be done to properly understand the ecosystem damage due to artificial light. Cities are expected to grow rapidly, exacerbating associated light pollution at night;  the situation requires urgent attention.  

What can be done to control Pollution by Artificial Light at Night (ALAN)?

Unlike other pollutants, ALAN is easily controllable - on a basic level, just switch off the lights and the effects of light pollution could be eliminated on a swift time scale.  However, life does not work that way - for generations, ALAN has delivered many benefits like extended work and recreational hours, and night activity is part of our lifestyle and culture - it is not possible to go back to preindustrial life.  But, there are many ways that we can reduce the harmful effects of ALAN on humans and rest of the ecosystem.  A mixture of good planning, design, education and common sense is required.   

It is important to realise that, until now, very little planning has gone into how we use outdoor lighting - 30 to 60% of energy consumed in lighting is unneeded.  This represents lost resources and also contributes to climate change. Many organisations throughout the world are actively promoting awareness about light pollution at night and working towards reducing the negative biological effects of ALAN - controlling ALAN saves money and it is good for health and the environment too - it is a win-win situation for everybody.  On an individual level, one needs to act to maintain a healthy circadian rhythm for your own health, and also consider ways to minimise the polluting effects of outdoor spaces. 

In order to reduce the polluting effects of artificial light at night, 

we can take some simple common sense steps to start with:

Outdoor Lighting:

1.  Turn off unneccesary lights: Most outdoor lights are switched on at dusk and stay on during the whole night. We do not really need street lights after about 1 am when both the road and pedestrian traffic is extremely low.  Most outdoor lights can be switched off for part of the night with significant cost savings as an additional benefit. 

Various studies have cast doubt on the security and crime reduction aspects of outdoor lighting.  Motion sensitive lights with surveillance cameras situated at appropriate places would be far more effective measures.  

2. Down-shield exterieor lights: Probably one of the most important measure one can  take to reduce skyglow, glare and trespass.  Most outdoor light fixtures in use emit light in all directions - there is no need to shine light upwards or even horizontally.  Properly shielded lights with reflectors can illuminate the desired area only and minimize light pollution by using much less energy and save costs.

3. Use high-efficiency warm white lights:  LED lights are super efficient and convert ~90% of electricity into light.  They are also controllable in the direction and intensity of emitted light.  LEDs do emit more blue light than the alternative high pressure sodium lamps.  By choosing a warm white temperature LED rated at < 2700K, one can increase the proportion of yellow and red wavelengths and reduce the impact of the light on humans and wild life.

4.  Use timers and motion-sensitive lights:  Motion sensitive lights only come on when needed - installing them can reduce  significant amount of light pollution and provide big cost savings without compromising safety and comfort.

5.  Optimise Light Intensity:  Light used should be at the lowest intensity required.  Again, timers can reduce the intensity of light during periods of low use (after midnight for example).  At present, lights that are too intense are used causing unwanted skyglow and glare.

Indoor Lighting:  Indoor lighting is most relevant for human health and with the introduction of economic-to-run LEDs, wasteful use of light has significantly increased.  Measures for reducing the impact of outdoor lighting are also applicable for indoor lighting - switching off lights in unoccupied areas, use of dimmers to reduce excessive intensity etc. - smart technology can be very effective. 

Most importantly one must avoid exposure to short wavelength blue light - for LED lights, the use of blue light blocking goggles at least two hours before bedtime is highly recommended as this ensures sufficient melatonin production for good quality sleep.

At night time, it is important to close the curtains, switch off porch & garden lights to ensure that glare and skyglow are not increased.   

Community Awareness:  The damaging effects of light pollution are not widely appreciated.  It is important to educate ourselves about these and help our neighbours and wider community to understand the issues involved.  In most areas, local authorities are responsible for the planning and installation of outdoor lighting.  Most measures to reduce light pollution save money, and it should not be too difficult to convince civic authorities of the merits of good design that provide optimum illumination levels at times of the night when it is required. 

Thanks for reading.  Please pass the link to this blog to friends, family and neighbours. 

Colour of Objects: Visible Light Spectrum; Primary Colours; Optical Illusions; How Do Colours Work?

 Blog Contents and Who am I?

Colours provide extraordinary detail and richness in the way we experience our surroundings.  Except for vision, our other senses only allow us access to the local environment.  Our ability to see objects with better than millimetre resolution and their colours has played a fundamental role in activities like foraging, finding mates, keeping healthy etc. and has guided our evolutionary development so much so that ~40% of our brain is devoted to vision related activity. 

The colour of objects is mainly determined by two factors; the light that illuminates them and the nature of pigments present in the object.  We look at these in detail in the following: 

Visible Light:  (For more details see)  Most of the energy from the Sun that reaches Earth is in the visible part of the electromagnetic (EM) spectrum - the visible spectrum extends from 400 to 700 nm (1 nanometre or nm is a billionth of a metre).  The human visual system has evolved to efficiently perceive the range of wavelength present in the visible spectrum.  Small amount of radiation in the ultraviolet (UV, 300 to 400 nm) and infrared (IR, >700 nm) also reaches the Earth's surface; some animals can indeed perceive UV and IR radiation.  

Human Perception of Colours:  Our vision has evolved to distinguish various wavelengths of light as colours - humans can perceive ten million colours.  Our eyes are most sensitive to light in the 500 to 600 nm range - perceived as green to yellow/orange colours - these are also the most abundant colours in the environment. Humans are trichromatic - the retina contains three types of colour receptor cells (called cones); see also. Colours are characterized by three properties - hue (H), saturation (S) and brightness (also called lightness L).  Links here (1, 2, 3) and slides in the appendix at the end of this blog provide more details.

Why we associate light of certain wavelength to a particular colour is not properly understood.  After all, light is simply a manifestation of varying electric and magnetic fields - its wavelength tells us how many cycles of such oscillations are happening per second (frequency = speed of light /wavelength).  The brain receives electrochemical  signals that are coded for the amount of light and the range of wavelengths that are incident on the retina. Why and how the brain translated these signals into particular colours is a mystery that is buried deeply into our evolutionary past.  

Also interesting is the way that the brain, after receiving the signals from the cones, decides on the colour of light that the cones had received.  For example, a wavelength in the region of 590 nm is interpreted as yellow, but so is a combination of two wavelengths of 523 nm (green) and 640 nm (red).    

Primary Colours:  We have learnt that white light is a mixture of wavelengths from 400 nm to 700 nm.  In fact, it is possible to generate white light by mixing light of three well-separated wavelengths, called the primary colours.  The most widely used primary colours are red, green and blue - the RGB system.  These are additive primary colours as the addition of these in equal proportion gives white light.  Adding RGB colours in different proportions generates other colours - an infinite variety of them.

Additive primary colours are generally associated with sources of light - lamps, bulbs, flames etc.  For example, you can mix light from two or more lamps to produce a different colour.

In real life, objects have characteristic colours when viewed in white light (e.g. during the day) - leaves are green, ripe apple is red etc.  Objects do not produce their own light but absorb, reflect and/or transmit light incident on them. Any light absorbed by the pigment in the object will subtract corresponding wavelengths from the white light - the resulting light will no longer be pure white.  The colour of the object is inferred by the eye-brain system when the reflected or transmitted light falls on the retina of the eye.  This discussion also leads us to describe the subtractive primary colours or complementary colours - these are shown on the slide above, but we now look at some examples to understand this complex subject.

We have established that the colour of an object that we see depends on the light absorbing pigments it contains and the wavelength of the illuminating light.  The slide shows how this manifests for a range of pigments and various colours of the illuminating light.  It demonstrates that white light allows objects to be viewed in the largest variety of colours.  Light sources other than white give only a limited description of the colour of objects.

Autumn Colours_ An Example of Changing Colours:   During the autumn season, many plants change colour from the summer green to yellow, orange and red, and present a remarkable colourful display.  The autumn colours are a consequence of changing pigment in the leaves.

During the summer plants receive plenty of sunlight and chlorophyll in the leaves uses solar energy to synthesize sugars (photosynthesis).  Chlorophyll reflects green light and gives the leaves their green colour.  Chlorophyll breaks down during photosynthesis but is constantly replaced during the summer period.

As autumn approaches, chlorophyll production shuts down and the green colour of the leaves fades.  This lets characteristic colours of other chemicals, already present throughout the summer but masked by chlorophyll, to show.  Carotenoids and xanthophylls reflect yellow and orange light and decay much more slowly than chlorophyll. Ash and birch trees change colour to yellow and orange first.

The pink, red and purple colours are due to the chemical anthocyanin which is synthesized in the autumn - bright sunlight and above freezing temperature help its production and give vivid colours in many trees like maples.

The pigment before the leaves fall is tannin and gives them a brown colour.  Tannin in oaks is the last pigment to breakdown.

When the Eye-Brain System Fails - Optical Illusions:  I  have mentioned that the eyes send information about the external world to the brain as electrical signals.  Such signals have gaps and only contain limited information - the three cones respond to a broad range of wavelength and do not provide a definite clue about the colour of the object. The brain then has the difficult task of interpreting this scrappy information into a unique result - not just about colour of objects but also their shape, size, motion and many other properties.  The brain relies heavily on past experiences and memory to accomplish this in a limited time of the order of milliseconds.  It is not surprising that sometimes the brain reaches a wrong decision.  How the eye/brain system works is discussed in the second half of my blog.  

It might be instructive to give a few examples of the brain's failure to detect the correct optical properties of objects in various contexts of colour, size, shape, motion etc.  To demonstrate this point, I have drawn some slides based on some popular optical illusions. (Click on the slide to see full page view)

An illusion that is easy to see is to stand on a bridge over a fast flowing river.  On looking at the water you feel as if the bridge is moving rapidly in a direction opposite to the flow of the river.  

How do Colours Work?  Colours affect our mood, feelings, emotions and behaviour; our reactions to colours are determined by a combination of psychological, physiological, personal, social and cultural factors.  For example, in fashion and marketing, colour impressions can account for 60% of the acceptance or rejection of products and services. In Western societies, white represents purity and innocence, but in many Eastern countries white is seen as a symbol of mourning.  It is fascinating to explore how different colours can have contrasting meanings in different cultures and contexts.  A detailed list of how the meaning of colours varies in different situations may be seen here.  

While it is well established that colours do influence the way we perceive the world around us, it is not clear to me how does it happen.  The question is how do colours affect our mood and behaviour? How do they work? The eye-brain system operates on a series of electrochemical (EC) signals and it is not at all clear why a wavelength of 640 nm is seen as red, while light of wavelength around 520 nm is  green.  This is probably a question that does not have an answer.  It is the brain's way to make sense of the world around us. 

Post Script:  This blog was written because many people I meet had wondered why the water in swimming pools is blue - also the striking blue colour of glaciers has been a source of confusion to many friends returning from trip to Alaska.  In order to explain this, it is important to set a background about colours and this is what I have tried to do in this blog.  Some fascinating physics is in play to give us the wonderful panoramic colours in the environment - the sky is blue during the day but at sunrise and sunsets it turns red or sometimes orange; clouds are white but grey as well, oceans are blue but look deep green sometimes etc.  In my next blog, I shall try to explain the science behind these various observations.

APPENDIX: 

The following slides explain the way we perceive colours - not as primary red, green or blue but in terms of their hue (tone), saturation and brightness.

please note an error in the normalised RGB value of the second colour square (70,70,125) should be normalised as (0.27, 0.27, 0.49).  The S value is calculated correctly.

Thanks for reading.  Comments are very welcomed.  Please pass on the link to this blog to friends and family. 

Most slides (except the first two slides) and text in this blog have been prepared by myself and you are welcome to use them freely but please  acknowledge this blog as 

https://ektalks.blogspot.com/2020/10/colour-of-objects-visible-light.html