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Refraction
Light, Sound and Waves
Refraction
for 14-16
You can show refraction in a ripple tank and model it by marching. Refraction is important in optics, but also provides a test of whether an energy carrier is waves or particles.
Demonstration
Shows refraction is caused when plane waves change speed as a result of a change in the depth of water in a ripple tank.
Apparatus and Materials
For the class
- Motor mounted on beam, with beam support
- Glass plate
- Nuts or washers, 4
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.
Photo-induced epilepsy
In all work with flashing lights, teachers must be aware of any student suffering from photo-induced epilepsy. This condition is very rare. However, make sensitive inquiry of any known epileptic to see whether an attack has ever been associated with flashing lights. If so, the student could be invited to leave the lab or shield his/her eyes as deemed advisable. It is impracticable to avoid the hazardous frequency range (7 to 15 Hz) in these experiments.
Read our standard health & safety guidance
The glass plates should be cut as shown in the diagram above. The nuts or washers act as raisers and should be about 4 mm thick.
The tank and the glass plate need to be very clean and free from grease.
To get sufficient change in wave speed at the boundary with the glass:
- Make the water on top of the glass very shallow.
- Pour water into the tank until it just covers the glass and then drain off a little of it. It may be necessary to re-level the tank to ensure that the film doesn’t break up into puddles.
- Use a low frequency, long wavelength (about 10 rev / second).
To get sharp waves, adjust the height of the vibrator so that it is just below the mean water level. The vibrator should just ‘hold up a film of water’ when it is still.
Procedure
- With the glass edge parallel to the vibrating beam, ask students to notice the difference in the speeds and wavelengths as waves cross the boundary.
- Adjusting the glass at various angles to the incoming waves, ask students to notice the directions of the refracted waves. You may want to use a couple of metre rules, to show the change in direction clearly.
Teaching Notes
- The waves travel more slowly in shallow water because of friction with the bottom.
- With the glass edge parallel to the vibrating beam, students should notice that the wave speed is reduced and wavelength becomes smaller as waves cross the boundary.
- With the glass edge at various angles to the incoming waves, students should notice that the waves change direction as they cross the boundary.
- What is the pattern? You may want to discuss the change in direction in terms of the ‘normal’. The waves bend towards the normal when their speed decreases (and away from the normal when their speed increases). You could go on to compare this with the bending of rays of light as they pass from air to water, perhaps referring to Snell’s law.
- Light: wave or particle? You may even want to conclude: if light consists of waves, the refraction of light shows that it must travel more slowly in water than in air, the opposite of the story for particles. Is refractive index the same for all frequencies? You could repeat 1 and 2 at a higher frequency to show that refractive index may depend on frequency (wavelength).
- How do lenses work? It is possible to use a piece of glass shaped like a biconvex lens in the ripple tank to show what happens to waves approaching a lens, though this is difficult to see clearly. It may be worth thinking about how the curvature of the wavefront changes entering and leaving the shallow area. In this way, you can build up a wave diagram and predict what will happen. What a lens does is change the curvature of wavefronts.
- Light - wave or particle? You may even want to conclude: if light consists of waves, the refraction of light shows that it must travel more slowly in water than in air, the opposite of the story for particles.
This experiment was safety-tested in February 2006
Up next
Marching model of refraction
Class Practical
A kinesthetic experience of refraction can help students to understand and remember what happens to wavefronts.
Apparatus and Materials
- Hard road with straight boundary adjoining soft grass, (alternatively, areas of asphalt marked with chalk can be used)
- Small army of students
- Discipline
Health & Safety and Technical Notes
Read our standard health & safety guidance
Procedure
- Students must first learn to march in step, with a uniform pace and then learn to change to steps half as long with the same frequency. Like a drill officer, count out ‘left-right, left-right’ until they are able to march in both ways.
- Align the army in fours or sixes, each tier with linked arms to imitate consecutive wavefronts. Then let them march on the road meeting the boundary obliquely. As soon as each crosses the boundary s/he must change to steps half as long. This will produce a change in direction of each wavefront, demonstrating refraction towards the normal.
- You may want to try the experiment again, with students marching from the grass to the road: refraction away from the normal will occur.
Teaching Notes
- This does take time to practise, but students are likely to enjoy it and so understand and remember why waves are refracted. On a wet day you could do this indoors, perhaps in a school hall or gymnasium.
- You could also make the army march through a converging
lens
drawn on the ground: the focal point becomes a pile-up of confusion.
Ed: We improved the text to this experiment following a comment from Robert Friedman.
This experiment was safety-tested in February 2006
Up next
Asking questions
Asking questions
Teaching Guidance for 11-14 14-16
Contrary to what is popularly believed, physical phenomena do not in themselves reveal theories. Interpreting what is seen often depends on knowing what you are looking for. There are many examples from the history of science either where a discovery was made as a result of the prepared mind of the scientist or where no progress was possible for a time because of theory-laden observation.
Avoid giving students instructions that tell them what they are going to see. With patience and care, even demonstration experiments can usefully model the questioning process basic to science. Students should have many opportunities for experiencing how a series of fruitful questions leads to understanding. A first question leads to an observation, which in turn provokes a new question, etc. Encourage students to discuss what they see.
This approach does take time, but is far better than simply giving dry answers before there is any grasp of a question. Students like to think for themselves and deserve to enjoy this pleasure. Passive learners are more likely to disengage.
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Using ripple tanks
Students should whenever possible experience and experiment for themselves using real equipment, rather than using software which shows ripples. They need to try things out for themselves rather than just following instructions.
Practical tips: See apparatus note "Ripple tank and accessories" for important details.
Asking questions – an activity which may help with discipline in a half-dark room – encourages students to think and extend their observations. When you ask whether the water moves along with the pattern, you could leave the students to devise their own tests and to think and experiment on their own, rather than giving detailed instructions.
It is worth considering where the dark and bright ripples come from. The convex and concave surfaces on the top of the wave make perfect lenses. When the light falls on the surface in the ripple tank then light is either focused by a convex surface or spread out by a concave surface. The concentrated light produces bright bands.
It takes time to set up ripple tanks properly. If you are going to use a set of ripple tanks for a class experiment, you may want to leave them on a side bench between successive lessons. Or, if the lesson follows lunch or morning break, you could ask a few students to come early and help set them up.
For demonstration purposes, you can now use a compact ripple tank designed to sit on an overhead projector. This produces a large image on screen, which the whole class will easily see.
Safety 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.
Photo-induced epilepsy:
In all work with flashing lights, teachers must be aware of any student suffering from photo-induced epilepsy. This condition is very rare. However, make sensitive inquiry of any known epileptic to see whether an attack has ever been associated with flashing lights. If so, the student could be invited to leave the lab or shield his/her eyes as deemed advisable. It is impracticable to avoid the hazardous frequency range (7 to 15 Hz) in these experiments. The danger is obviously greater for xenon stroboscopes than for hand ones.
Up next
Using wave simulations
There are many excellent applets available online that show wave behaviour as if observing a ripple tank or oscilloscope screen.
These cannot substitute for experience of the phenomena themselves but provide a powerful way of helping students to visualize. They provide a valuable complement to experiments by removing extraneous effects.