Light and its effects
Sources and the speed of light
Life
as we know it would be totally different without light, so we will start by seeing where light
comes from.
The earliest form of artificial light was fire - produced by burning wood, oil
or candles.
Today light is normally produced by
electricity, either by heating metals as in a filament lamp or by making gases glow as in a
gas filled tube. The big difference is that the gas filled tubes are much colder than the
filament lamps, and so are much more efficient – less energy is wasted as
heat.
Light travels very fast, to us it seems that its speed is infinite but the speed of
light can actually be measured. For example in space light travels at 300 000 000 m/s! This
means that a pulse of light can travel the 450 000 km to the Moon in 1.5 seconds! Nothing
can travel faster than this; it is a kind of cosmic speed limit.
Speed of light in space = 300 000 km/s = 300 000 000 m/s = 186 000 miles per hour
However the distances in astronomy are so huge that
even light takes some time to cross the vastness of space. It takes light just over 8 minutes
to reach the Earth from the Sun, almost 4½ years from the nearest star and the light from the
Andromeda galaxy that is reaching the Earth today started its journey some two million
years ago!
Astronomers use the very large speed of light to measure distance. They use
the LIGHT YEAR — the distance light travels in one year. This is about 10 000 000 000 000
km! On this scale the Andromeda galaxy is said to be 2 million light years away.
Example problem
Calculate the distance of the star Sirius if light takes light nine years to travel from Sirius to the Earth.
Distance = speed x time = 300 000 x 9 x 365 x 24 x 3600 = 85 000 000 miliion km
Reflection, transmission,
absorption
When light hits a surface one or more of the above things
happens.
(a) Reflection - the light rebounds, regularly if the surface is flat but
irregularly if it is not. This irregular reflection scatters the light and is called DIFFUSE
reflection.
(b) Transmission - the light passes through the material. If it goes through
regularly the material is TRANSPARENT, if the light is scattered it is
TRANSLUCENT.
(c) Absorption - the light is stopped by the material - it is
OPAQUE.
Images in light
Mirrors, lenses and optical instruments
produce images of objects. A pinhole camera is a simple way of making an image on a
screen. There are two types of image -
REAL and
VIRTUAL.
A real image is one that can be formed on a screen while a virtual image is one that is formed in space, the light seems to be coming from that point.
Light rays and waves
If you look at the sunlight shining
through the leaves of a wood on a sunny day, or go to the pictures and watch the projector
beam shining though the dust in the air, you can see the path of the light. We call this path a
RAY of light, sources of light seem to give out thousands of rays in all directions. You will
also notice that light rays travel in straight lines. We will use this fact in all our work on simple
effects of light.
Light rays travel in straight lines
We now know that light travels as waves. These waves are transverse, which means that the vibrations of the wave are at right angles to the direction in which the wave is travelling.
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