|When light goes through a prism, why does it exit
like a rainbow?
|Question Date: 2014-05-20|
White light entering a prism is actually made up
of many different kinds of light. Lights of
different colors have different "wavelengths"
(this is the distance between the peaks of two
here about waves).
Our eyes sense the different wavelengths and we
see this as color. When white light enters the
prism, each kind of light within white light (all
the colors) are reflected at different angles
because of the different wavelengths. This
effectively separates all of the colors present in
white light into different bands which we can see
as the spectrum. (prism
monochrometer) What's really cool is that by
using two prisms, if you have two prisms, you can
use one to separate the light into the colors, and
another to put it all back together into white
This is a great question! When white light passes
through a prism, the prism disperses the light
into what is called its constituent colors. Prisms
are able to do this when they are transparent and
have flat surfaces with some angle between them.
When light travel through the air hits the prism,
it changes its speed, and is thus enters the prism
at a new angle (it's "refracted"). This angle
depends on the refractive index of the material in
the prism, which can vary with wavelength of
light. Wavelength of light in turn, corresponds to
different colors of light. So these different
colors will travel at different angles in the
prism and exit as a "rainbow" or spectrum of
colors, rather than as white light.
The process happens because of the way that waves
work. Light travels at different speeds depending
on what it's shining through. In a empty space,
light travels at about 300,000 kilometers/second,
which is the fastest speed, but in a substance
like glass or water it moves slower. Also,
different colors of light move at different speeds
in different materials, and this movement at
different materials causes the light waves to
separate out into colors, which is why you see
rainbows. Prisms work just like rainbows, except
that the substance is glass instead of drop of
"White" light from the sun is made from the
combination of many colours of light. When light
interacts with matter, the strength of the
interaction depends on the colour (the energy) of
the light. So when white light (of many colours)
enters a prism, the different components interact
with different strengths.
What this means is that blue light will be bent
differently than green light, than red light, and
so on. This causes the many colours of light to
separate, and is why you see a rainbow of colour.
This happens in a prism, but also happens very
notably in water droplets, and this is what leads
to rainbows in the sky.
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