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

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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.

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

Net-Zero by 2050 - How does One Eliminate Emissions from Agriculture, Heavy Transport & Industry? _A Community Education Feature

 Index of Blogs and Courses

Urgent and world-wide action is required to make transition to clean energy.  Business as usual will bring catastrophic environmenal and ecological changes for future generations.

UN warns that world risks becoming 'uninhabitable hell' for millions unless leaders take climate action (October 13, 2020)

In a recent talk, I had discussed the impact of global warming and climate change on our planet.  Dire as they are, it is difficult to feel hopeful that the nations of the world will act in time to mitigate the worst effects of climate change.  Doing nothing will simply make the problems worse with catastrophic consequences - we need to do our best to limit climate change.  The goal (2⁰C rise) and aspiration (limit the rise to 1.5⁰C) of the 2015 Paris Accord require that by the year 2050, CO2 net emissions are brought down to zero (Net Zero) from the current levels of about 36 gigatons per year and significantly reduce the emission of methane and nitrous oxide . Five years on, the progress made has been disappointing; emissions have continued to increase and the scientific opinion is converging to the view that by 2100, global warming will be in the region of 3⁰C - a serious and frightening prospect. The following slides summarize the situation:

In their mammoth 2019 report (630 pages), IPCC have looked at various pathways for limiting global warming to 1.5⁰C.  Essentially, there is a lot of CO2 in the atmosphere already (411 parts per million - October 2020) and we continue to add more CO2 every day (over the past decade CO2 levels have increased by 24 parts per million or 5.8%). All pathways require that we start to cut down CO2 emission now or in the very near future with Net Zero emissions by around 2050, and also remove CO2 from the atmosphere at a rate depending on how soon and how fast we cut down emissions. The later we start, the more severe the reductions will have to be.  In the next slide, I show one of the pathways that IPCC have analysed - these are theoretical possibilities and assume certain modest technological developments will be forthcoming. Please read the report when you have a week to spare.

Cutting Emissions is Hard:  Many studies have analysed scenarios to achieve net-zero by 2050.  Most studies infer that achieving net-zero is possible with modest advancements in current technologies requiring some R&D effort.  The costs are manageable too - ranging from 0.5 to 2% of the GDP.  The scientific, technological and economic feasibility of achieving net-zero is convincing - the question is (has been over the past decades) - do the nations have the political will to act? Will the world rise to the challenge?  So far the progress in reducing emissions has been disappointing.  

Let us look at some of the reasons that make it difficult for nations to adopt strong abatement policies.

1.  Energy demand is continuing to increase.  Our standard of living depends directly on the amount of energy we use. In rich countries, most people will not agree to sacrifice the way they live.  In developing countries, people aspire to live 'better' and that translates into increased energy consumption.  Additionally, population is expected to be greater than 10  billion by 2100, up from 7.8 billion now.  Politically, national governments find it difficult to plan for any reduction in energy consumption.  Also most governments work on debt funded economic models, underpinned by perpetual growth - more consumption is encouraged by businesses.

2.  Fossil Fuels produce most of the energy just now:  About 80% of the global energy production uses fossil fuels which emit copious amount of CO2 into the atmosphere. Renewables (solar & wind)  generate a meagre 2% of the global energy.  With vested interests preventing change, the task of replacing fossil fuels by renewable energy is non-trivial. 

Fossil fuels also produce 64% of the electricity with hydroelectric, nuclear and renewables (solar and wind) making up the rest.

It may be that electricity generation by renewables (mostly solar and wind) can be ramped up to such an extent that we can have near 100% decarbonised electricity.  A factor of 10 increase in solar and wind installations will be required to replace fossil fuels in current electricity generation.  However, note that electricity is less than 20% of the total energy needs of the world which are largely met by fossil fuel combustion; solar and wind currently supply only about 2% of the total energy needs. Nuclear and hydro-electric energy generation capacity is not expected to increase much and likely to stay near current level for the foreseeable future.

Another important reason for not burning fossil fuels is that they produce large amounts of polluting gases and particulate matter - the air pollution is estimated to cause 7 million premature deaths each year.

3.  Harder to Abate Sectors:  Besides meeting current electricity demand, renewable energy can also replace fossil fuels in light transport (electric cars), much of agricultural machinery, space heating and light-weight industrial plants.  

However, it is not cost effective to fully electrify heavy transport (shipping and aviation) and the manufacturing of cement, iron and steel. Plastic industry uses lot of fossil fuels too. These are 'Harder to Abate' sectors as the technology to replace fossil fuel generated energy by electricity either does not exist (aviation is a good example) or is relatively inefficient. The emissions produced by different sectors are shown:

The following slide is a summary of figure 3 in Davis et al.

Another hard to abate sector is agriculture (AFOLU), particularly animal farming where cattle numbers have increased extremely rapidly.  They generate large amounts of methane (enteric fermentation).  Globally, ruminant livestock produce about 3.3 GT CO2 equivalent of enteric methane emissions. This contribution is expected to increase in future years. See also where it is estimated that humans and farm animals exhale in excess of 8.5 GT CO2 per year - this is a large contribution to overall emissions and difficult to reduce.  

What Are Our Options?  

1. Energy efficiency:  We can drastically reduce the energy we use (increase energy efficiency - this means that we cut down the amount of energy to perform a task). It is also important to make serious move to a circular economy with zero waste.  It is estimated that energy efficiency gains can reduce  energy requirements significantly and it is possible to keep the energy demand in 2050 at 2018 levels or lower!  (See Slide below) This will certainly help limit CO2 emissions and must be seriously implemented.  However, as long as fossil fuels are in use, CO2 concentration in the atmosphere will continue to increase.  

Energy efficiency on a personal level - keeping the central heating lower by 1 or 2⁰C, switching off lights in unoccupied rooms, walking or cycling instead of using car for short distance journeys - can be very effective.  On a community and national level, much can be done to help - for example, heat pumps provide a factor of three savings in energy.  Most buildings could be heated or cooled using heat pumps with a reduction of up to 70% in CO2 emissions from this sector.  The ultimate goal will be to run the heat pumps on renewable energy and completely eliminate CO2 emissions from space heating and cooling.  

2. Renewable energy:  Massively expand existing renewable energy generation to replace fossil fuels.  Remember, nuclear and hydro energy have extremely limited growth potential. Solar and wind energy is already cheaper than fossil fuel energy and further price reductions are expected.  Over the next 10 years, it will make good economic sense to replace much of fossil fuel use (wherever possible) by expanding renewable energy generation.

3. Hydrogen economy: Use renewable energy generated hydrogen (green hydrogen) to replace the high energy density fossil fuels in harder to abate sectors.  Hydrogen is basically a way to store primary energy produced either by fossil fuels or solar/wind or biomass. It can be transported over short distances, but can also be used to synthesize other high energy density fuels (ammonia, methanol and ethanol) for long distance transport, chemicals and in iron & steel industries.

Currently, most of the 75 million tonnes (in 2019) of hydrogen is obtained from steam reforming of natural gas (grey hydrogen), and is used in fertilizers, refining petrochemicals and in the production of ammonia and methanol in the chemical industry.  Hydrogen production using fossil fuel energy added 0.85 GT of CO2 to the atmosphere in 2019.  

It is expected that by 2050, the emission free production of 500 million tonnes of green and blue hydrogen will displace 1400 Mtoe (million tonnes of oil equivalent) of fossil fuels to generate 17000 TWh of energy. (1 kg of hydrogen generates 33 kWh; 1 kWh = 1 unit of electricity).

4. Biomass energy:  Biomass is any organic, biological or plant matter (excluding fossil fuels) which can be converted into a usable energy source; for example, trees, plants, wood, grasses, organic wastes etc.  In biomass, energy is stored in the form of carbohydrates (cellulose, starch and sugar) that are produced in the photosynthesis of solar energy by plants. Biomass is a renewable energy source; however, as is often claimed, it is debatable if biomass is net emission free (carbon neutral); or if biomass with carbon capture, use and storage (CCUS) is a negative emission energy source.

Essentially, plants use atmospheric CO2 to make carbohydrates – the CO2 is released back to the atmosphere when biomass is used to generate energy.  New plant growth will repeat this process – renewable energy source.  However, how land is used to cultivate plants for bioenergy is critical for biomass to be carbon neutral.  Additionally, the processing of biomass to synthesize biofuels requires energy that is currently supplied by fossil fuels.  This makes the lifecycle biomass energy a net GHG emitter.  A good example is the synthesis of biofuels from sugarcane or grains which are generally grown on crop lands or by clearing forests. This not only removes carbon sinks from the environment (creates biofuel carbon debt) but also adversely affects land availability for agriculture. In contrast, biofuels made from waste biomass or from biomass grown on degraded and abandoned agricultural lands planted with perennials incur little or no carbon debt and can offer immediate and sustained GHG advantages.

Notice that used cooking oil is much better than other biomass because it does not require any land use.

 5. Carbon capture: Capturing CO2 produced in industrial  plants using fossils is an expensive process and significantly reduces their energy production efficiency. Also, CO2 produced by diffused sources like automobiles, planes, farming etc. is very difficult to capture at source.  This does not affect the CO2 already present in the atmosphere.  As we have discussed above, some CO2 emissions, like CO2 exhaled by humans and farm animals, will be impossible to abate and it will be important to capture CO2 from the atmosphere to achieve net-zero.

We need to develop carbon capture, use and storage technology with aim to remove, by the year 2050, about 10 gigatonnes of CO2 from the atmosphere per year. 

The best hope seems to be direct air capture (DAC) technology where the atmospheric air is pumped through a system with special filters to absorb CO2.  On heating, the filters release the CO2 that can be used in industrial processes or stored as carbonate salts in geological depositories.  The following slide explains the process:

The current cost of DAC is of the order of $400 per captured tonne of CO2.  To capture 10 GT CO2 per year will cost 4 trillion dollars!  DAC is a new technology and it is expected that cost will substantially come down in the future.

Reforestation:  Trees, through photosynthesis  absorb CO2 from the atmosphere and store it as wood.  Planting new trees is a great idea for helping to reduce CO2 atmospheric concentration.  1t.org   mission is to plant one trillion trees by 2030 to combat the worst effects of climate change.  However, one notes that trees take time to mature and the process is too slow to meet the goal of Net-Zero by 2050.  Reforestation is a great idea and will help in reducing CO2 conecentrations.

BioEnergy with Carbon Capture and Storage (BECCS): The idea  is to grow crops that soak up CO2 and then burn them to produce energy but capture and store all the CO2 that is produced.  To meet the Net-Zero goal, we shall need to deploy huge amounts of land which would affect agricultural production at a time when global population is increasing.  It is estimated that BECCS could cause an increase in food prices by 3 to 5 times by 2050 - an unacceptable burden on the world's poor nations.   

6. Transition to plant protein:  Make a rapid transition from farm based meat/protein to plant protein. At present, there is no other way to reduce the enteric emissions from cattle whose numbers have been increasing rapidly over the past 50 years.  Switching from animal to plant protein will also directly reduce the emissions from animal farming.

Nothing will benefit health and increase chances for survival of life on Earth as much as the evolution to a vegetarian diet.          ...Albert Einstein (quoted)

AFOLU  sector is a big emitter of GHGs and rather difficult to abate. A 2020 report by McKinsey has analysed the options in detail and the slide summarizes their findings:

Where do we go from here?

The above list is a big ask but is doable - given the will and co-operation of the nations of the world. Renewable electricity is already competitive with fossil fuels generated electricity despite the 5+ trillion dollars of subsidy that fossil fuel industry enjoys.  Much of the basic technology exists, and can easily be developed further to accomplish full decarbonisation of the industry. The time window to start is very narrow - may be one or two years.  We have known about climate crisis for decades but have failed to act.  Action at global level is required and it is difficult to be optimistic that we shall be able to limit global  warming to 1.5⁰C or even to 2⁰C.  

Only six countries have legally binding position to achieve net-zero (click here for the list + China) while some important polluters (e.g. the USA) have effectively given up on it. Despite announcing their goal to reach net-zero by 2060, China is going ahead with the building of new coal power stations - it is not clear how serious these commitments are.

India expects to increase its coal-fired electricity generation by 22% over the next three years.  Such facilities have an operational life span of 20 to 40 years.  Demand of electricity is increasing rapidly in India and one can be sympathetic to the need to increase generation capacity quickly.  This sort of situation just highlights the result of failure to act in time.  With proper R&D support renewable energy could have been developed quicker and is the obvious choice for generating additional power - particularly in a country like India where plentiful solar energy is available.

Concluding Remarks:  It is important to meet the net-zero target.  This is scientifically/technologically possible but requires serious effort by all countries of the world.  The cost of abatement may be as much as 2% of global GDP (~3 trillion dollars per year); this is  affordable.  In absence of effective action, soon it may be difficult to meet the 1.5⁰C or 2⁰C  target but that will make the world a much less pleasant place to live - this is the only planet we have.

Thanks for reading.  Any comments will be appreciated.  Please pass on the link to this blog to your friends and family.