Friday, 10 December 2010

On the Nature of Light: Wave-Particle Duality


Three fundamental quantities that help us to perceive the world around us

Space 3-D space - We can use a ruler to measure this
Time - Use a clock - atomic clocks can be accurate to 1 sec in a billion years
Means of transmitting information – LIGHT or EM waves in general…..

The speed of light is the fastest speed at which information/energy can travel
Speed of light is 300,000 km per second
The finite speed of light means that the time we receive the signal is later than the time the signal left the source (we are observing what happened in the past). This situation may be different for different observers and can create some bizarre effects.
Light plays a fundamental role in the theory of relativity.

Puzzling behaviour of light: Sometimes light acts as a wave while at other times it behaves like a stream of particles (photons) - Light displays a dual nature

But first a brief history:
1675 - Isaac Newton thought that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. Newton Published Optiks in 1704
Despite some serious problems with this view, Newton’s reputation helped the particle theory of light to hold sway during the 18th century

Robert Hooke (1635 - 1703) proposed in 1660 a wave theory of light.
Christiaan Huygens (1629 - 1695)in 1690 suggested that light was emitted in all directions as a series of waves in a medium called the Luminiferous ether
Thomas Young (1773 – 1829) in 1801 performed the Double-slit Interference Experiment.
Some say that it is the most beautiful experiment in physics.

The observations of the double-slit experiment could only be explained if light behaved as waves...

In 1860s, Maxwell combined the fields of electricity, magnetism and light and predicted the existence of electromagnetic (EM) waves. In his brilliant and very successful theory, Maxwell showed that all EM waves travel at 300,000 km/s which was the same as the measured speed of light.
Light is the “visible” EM wave
EM waves span a wide spectrum from radio waves to nuclear gamma rays


It was thought that as a wave motion, light would require a medium to travel - such a medium was called ether which was supposed to pervade the whole universe and had some really bizarre properties like being extremely stiff to allow vibrations of very high frequencies, be extremely thin (rare) to allow unimpeded motion of planets and objetcs through it etc. Attempts were made to measure the speed of the Earth relative to the ether but even the most sensitive experiments failed to find any evidence of the expected relative motion.
This caused a big headache for physicists at the end of the 19th Century.
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