Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Refraction of particles and of waves

Demonstration

Particles and waves refract differently. Only waves refract in the same way as light.

Apparatus and materials

For the class

Ripple tank and accessories

Hinged platform made from two pieces of hardboard

Launching ramp

Paper, white and soft carbon paper, of 2 metre rules

Glass plate to place in ripple tank

Health & Safety and Technical notes


Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.  

Place the power supply for the lamp on a bench, not on the floor by the tank.

An alternative to carbon paper and plain paper is to line up two-metre rules along the path of the incident and refracted ball bearing.

 

Procedure


hinged platforma Raise the larger piece of hardboard on a block (or book) as shown. Place carbon paper over a sheet of white paper both on the upper platform and on the bench-top. The edges should be accurately parallel to the edge of the slope. 

b Launch the ball bearing down the ramp. Try different angles of incidence. Its path will be refracted ‘towards the normal’ as it speeds up in crossing from the upper platform to the bench-top. 
 
c You may also try launching the ball on the bottom surface to show refraction ‘away from the normal' as the ball slows down on going up the ramp. For this the height of the platform above the bench will need to be reduced to about 1/2 cm.


Teaching notes


1 Ask: Do particles refract the same way as light? 
Mention that it has been found experimentally that the speed of light in glass is less than the speed of light in air. Discuss similarities and differences between the behaviour of particles and light.

2 Ask Do waves refract in the same way as light? 
Repeat the experiment in a ripple tank, using a shallow area created by placing a glass plate in the tank to slow down the waves. (See also Refraction of ripples entering shallow water.) 
 
3 Students should see clearly the changes in direction as the ball bearing changes its speed, for example bending towards the normal when it rolls down the ramp. Point out that the apparatus does not produce an immediate change in speed, but a gradual increase as the ball bearing rolls down the ramp. You can also show 'total internal reflection' with this apparatus. 
When a ray of light enters a block of glass, the ray is bent towards the normal. So, the particle model for light requires the speed to increase in glass. Other experiments show that light travels more slowly in glass, so the particle model fails the 'velocity test'. The particle model also has trouble explaining the way that light is partially reflected and partially refracted at some boundaries. 
 
Waves bend towards the normal when they slow down, just as light does. And both light and waves bend away from the normal when they speed up. 
 
This experiment was safety-checked in February 2006