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Aim: to Verify Snell's Law of Refraction and to Estimate the Speed of Light inside a Transparent Plastic Block
See Refraction: Snell's Law, Total Internal Reflection
 
Method
Direct a narrow beam of light towards the plastic block at a range of angles of incidence.
Measure the angle of refraction corresponding to each angle of incidence.
Try to find a semi-circular block of plastic, as shown below.
This is a convenient shape to use because, if you direct the beam of light towards the centre of the semi-circle, it will always meet the curved side at 90 and so will not be refracted there.
Ideally the block is placed on a rotatable support marked out in degrees as shown here but if you can't find that you can always put the block on a large sheet of paper, mark the paths of the light and measure angles later.
 
Adjust the position of the light source carefully to ensure that when the angle of incidence is zero, the angle of refraction is also zero.
 
Plot a suitable graph to verify Snell's Law.
Use the graph to find the refractive index of the plastic and hence calculate the speed of light inside the plastic, remembering that the speed in air is about 3108ms-1.
 
If you rotate the block through 180, more fun can be had finding the critical angle if incidence for the plastic/air interface.
If you gradually increase the angle of incidence, by rotating the block slowly, you will come to a point when the refracted light disappears...
 
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