Finding out the Speed of Light Through Perspex

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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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