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Light, Sound and Waves
Waves guidance notes
for 14-16
Waves guidance notes.
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.
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Classroom management in semi-darkness
There are some experiments which must be done in semi-darkness, for example, optics experiments and ripple tanks. You need to plan carefully for such lessons. Ensure that students are clear about what they need to do during such activities and they are not given unnecessary time. Keep an eye on what is going on in the class, and act quickly to dampen down any inappropriate behaviour before it gets out of hand.
Shadows on the ceiling will reveal movements that are not in your direct line of sight.
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Why experiment with waves?
A wide range of mechanical and electromagnetic waves become understandable with a limited number of concepts, e.g. interference.
Practical experience of waves builds familiarity with wave phenomena on which future experience can be based.
Wave phenomena provide many interesting demonstrations and experiments which students can enjoy doing themselves.
Students are able to do the experiments fairly safely, designing their own investigations, provided a supply of equipment is available, on a ‘cafeteria-like’ basis.
Many demonstrations require the development of skills for the effect to be clearly seen, and the acquisition of these skills is a pleasure in itself.
Photograph courtesy of Lucy Hollis
In one of his autobiographical essays, Richard Feynman recounts his experience of finding that degree-level students were unable to connect optics theory with optical effects seen outside the classroom window. Students have many opportunities to observe waves and ripples for themselves. This photo shows diffraction of ripples as they pass an obstruction in a pond.
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Light - waves or particles?
What is light?
Two thinking models of light were argued about for many years. In the 17th century, Isaac Newton decided in favour of a particle theory because this would account for straight rays and sharp shadows. Around the same time, Huygens developed a wave theory. Much later (about 1800) the wave model of light gained strong experimental support from the work of Thomas Young.
There were two serious difficulties with Newton’s particle theory. It failed to explain:
- the fact that when a beam of light passes from one medium to another, some of it is reflected and some of it is refracted.
- the phenomena of interference and diffraction.
These two difficulties forced Newton to suggest a strange scheme that endowed the interface with alternating ‘fits’ of easy reflection and easy transmission. He knew very well that this implied some periodic activity connected with moving particles, so that one could assign a wavelength yet he did not change to a wave theory, because he considered sharp shadows too difficult to reconcile with waves.
Long after Newton’s time, the speed of light in water was measured and compared with the speed of light in air. Light travels slower in water. That is generally regarded as a crucial experiment which decided clearly against the particle theory. Yet most crucial experiments, if not all, are only crucial, leading to an inescapable decision: if one sticks to the full details of the theories being tested.
Newton’s prediction assumed that the mass of the light-particle remains constant, and that its component of momentum along the interface is conserved-by symmetry and Newton’s second law. On the basis of constant mass that predicts greater speed in water. If we allow the particle to change its mass but conserve the energy stored kinetically, the prediction is reversed; smaller speed in water. Thus not even here is there a fully crucial test unless we choose a particular set of assumptions