|
Refraction of light
When light changes direction when it passes from one material into
another, it is said to be refracted. Refraction can be observed by putting
a pencil in a glass of water and looking at it from an angle above the
rim. The pencil appears to be in two separate pieces, above and below
the surface of the water. Refraction is caused by light travelling more
slowly in water than in air. Many everyday experiences with light, like
the sparkle of a diamond and rainbows are caused by refraction.
|
Refraction
through a prism
It was Sir Isaac Newton who first conducted an experiment
to demonstrate that white light was really made up of a number of
colours. He used a glass triangular prism and a beam of light coming
through a small hole in a shutter on his window. He was able to
project an array of colours onto the wall and identified them as
the seven distinct colours that we know as the visible spectrum.
He went further though to demonstrate that each colour was indeed
a separate part of the spectrum. To do this he placed another triangular
prism in the line of each colour and found that there was no more
colour change.
|
 |
|
Colours of the rainbow
 When
the Sun shines from behind, and there are rain clouds ahead, you
can sometimes see a rainbow. This is caused by white light being
refracted by the raindrops, but this time being reflected back to
the eye as well. The colours of the rainbow are red, orange, yellow,
green, blue, indigo and violet. These colours can be remembered by
the first letters of a sentence like: Read over your good book in
vain. There are other sentences too, and you might know different
ways of remembering the sequence.
|
Chromatic aberration
Because lenses work by refracting light, they also
bend different colours through slightly different angles. This means that
unwanted colours sometimes appear at the edges of bright parts of the image.
Quality cameras and telescopes overcome this aberration by using combinations
of lenses to counteract the effect - these are called achromatic lenses.
This same chromatic aberration can also be seen at the edges of bevelled
mirrors.
Sparkling diamonds
Refraction can be put to good use. Refraction is taken
into account in the cutting of diamonds, or crystal glass. One of the
reasons why these materials can be made to sparkle is that they have a
high refractive index. Refractive index is a measure of how much the light
changes direction as it passes from one medium into another. To maximise
the refraction from diamond and crystals, it's important to make cuts
at specific angles so that the light is refracted and reflected many times.
A dull diamond is transformed into a sparkling jewel by an expert cutting
up to fifty-eight sides, or facets.
|
|
Refraction through a prism
