Finding out the Speed of Light Through Perspex
Aim
To find out the speed of light through Perspex by passing a narrow ray
of light through a D-Block of Perspex, by using the same concepts and
ideas as Snell's Law.
Background
Light is an electromagnetic wave. The speed of light depends on the
medium through which it propagates: it goes fastest in a vacuum,
almost as fast in air but considerably slower in glass. Because of the
special role it plays in many parts of physics, the speed of light in
a vacuum has been given its own symbol: c. The speed of light in any
other material we denote with v. The ratio of the two is defined as
the refractive index, symbol: n.
Equations
Refractive Index
Sin I
Speed of light in Perspex
=a constant =
Speed of light in light in air
Sin R
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I could also use my graph to calculate the refractive index
Apparatus
* Ray Box
* Perspex D-Block
* Protractor paper
* Pen/ Pencil
* Ruler
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Diagram
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Prediction
My Prediction is that first of all the ray of light will travel in a
straight le towards the normal inside the glass prism. Then on leaving
it will refract away from the normal. The effect of this is that the
emergent ray is parallel to the incident ray, but is "laterally
displaced" from it.
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Info for prediction
We know the speed of light in air which is 300,000,000 m/p/s, so
firstly work out what sine I over sine r is and you multiply
300,000,000 by what ever you work out sine I over sine r to be. E.g.
if refractive index = 0.7 you would do 300,000,000 x 0.7 =
210,000,000, so speed of light in Perspex is 210,000,000.
This is a sketch of what I expect my final Graph to look like:
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