Light is a small, but important part of the electromagnetic (EM)
spectrum. The EM spectrum ranges from cosmic gamma rays at short
wavelengths (3×10-15 m) to electromagnets at long
wavelengths (3×108 m), with light at the shorter
side (5×10-9 m) on a logarithmic scale.
The response of the human eye roughly matches that of the Sun viewed
from Earth. What we call light depends on the overall sensitivity
of the eye, ranging in wavelength from 380 nm to 760 nm.
What we identify as colour depends on the wavelength sensitivity
of different receptors in the eye and the way this information is
processed in the brain. For further discussion on colour, click here.
Light and all EM radiation are composed of small parcels called
photons. Photons are now thought to be carriers of EM force. They
are called parcels (or packets) rather than particles or waves because,
depending on their interactions with other matter, they have either
particle or wavelike behaviour. This duality in the nature of photons
is a key aspect of Quantum theory. For further discussion on the
nature of light (and photons), click here.
The speed of light (and other
EM radiation) in a vacuum, usually given the symbol c, is
3×108 m/s. This speed does not depend on the observer
(the observer's frame of reference); in this, light is different
from normal human experience where objects appear faster or slower
depending on our own speed. This special behaviour of light is the
basis for the Special Theory of Relativity.
once said, "Time is what prevents everything happening at once".
The finite, fixed speed of light in vacuum marks this Time.
The amount of energy E carried by a photon is given by E=hn
where h is Planck's constant and n (pronounced nu) is frequency. Frequency (the number of oscillations
per second) is speed divided by wavelength.
The speed of light is not constant in different materials. It is
always slower in a gas, liquid or solid than in a vacuum. Since
the photon's energy doesn't change, its frequency also cannot change,
and therefore its wavelength changes. The wavelength decreases as
the speed decreases.
The speed of light in air changes with the temperature and pressure
and humidity of the air, not enough perhaps for our eyes to detect,
but enough to upset sensitive optical instruments. More about this small shift in wavelength in air can be found here.
For a discussion of
- shadows and reflections
- refraction in a pool
- double images of sun and moon
- seeing underwater
- how a rainbow works
- why the sky is blue
- colours of sunrise and sunset
- luminous plants, animals and stones
- and many other fascinating things about the light
including some truly beautiful pictures, see
Minnaert, Light and Color in the Outdoors, Springer (1974).
For an entertaining history of light and the many
inventions that accompany it
- electric lights
- The Light Fantastic, Penguin (1981),
by the sadly missed Peter Mason.
First published on the web: 15 December 1999.
Author: Richard Payling