Snell's Law

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Snell's Law

Snell's Law:

sin i v1

========

sin r v2

v1 is a constant known as the refractive index (m), using this we can

rearrange the

v2 equation to form:

sin i = msin r

This means that sin i is directly proportional to sin r and if we

measure both of these we will be able to calculate the refractive

index.

[IMAGE]

It can then be seen that once the ray enters the prism, it does not

continue in a straight line, rather it is abruptly deviated and then

travels through the prism until it reaches the far face, where the ray

again suddenly deviates as it moves back into the air. This emergent

ray is seen to be parallel to the incident ray.

The abrupt change in direction of the ray of light as it changes the

medium in which it is travelling is called refraction. It arises

because light travels at different speeds in different media. The more

(optically) dense the medium, the slower it travels. This means that

light travels faster in air than in perspex. The direction of

refraction is also important here. As the ray of light slows down when

it enters the glass, it deviated towards the normal, while as it

speeds up as it leaves the perspex, it is seen to deviate away from

the normal.

By measuring the angles i and r we are able to calculate the

refractive index of Perspex.

Method:

Equipment:

Light box

Power Pack

Perspex block

Lens

White paper

Pencil

Enlargement of a protractor

* Set up the equipment as shown above.

* Place the Perspex so that the edge of it is in line with the edge

of the photocopied protractor.

* Position the light beam so that it strikes the Perspex at the

centre of the protractor.

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