Refraction
Light, Sound and Waves

Reflection and refraction of particles

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The world is bathed in light. But what is light, and how does it travel from one place to another? These experiments demonstrate the power and limitations of physical models, while also introducing the two contenders, waves and particles (corpuscles). 
 
Most teachers will want to do these as participative demonstrations, accompanied by considerable discussion - possibly including the history of relevant ideas and crucial experiments.

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Particle model of reflection

Refraction
Light, Sound and Waves

Particle model of reflection

Practical Activity for 14-16

Demonstration

Both particles and waves reflect in the same way as light.

Apparatus and Materials

For the class

  • Glass block
  • Bouncing ball
  • Marble
  • Paper, white
  • Carbon paper, soft

Health & Safety and Technical Notes

Read our standard health & safety guidance


The block may be so massive that it does not move on collision. If not, place a large weight behind it or fix it to the bench with a G-clamp.

Procedure

  1. One student bounces a ball on the floor, at various angles, across a short distance to a partner. The paths before and after collision are in a vertical plane.
  2. A student rolls a marble along a table (in a horizontal plane) to gently bounce against a vertical surface of a glass block. Repeat the experiment at different angles of incidence.

Teaching Notes

  • Introduce these quick and simple experiments by asking,
  • "Do particles reflect in the same way as light?"
  • In step 2 you can record the path of the marble before and after hitting the wall by placing a sheet of carbon paper over a sheet of plain paper on the table. Ask students look for a pattern in the angles. It is easy to see the symmetry of the situation, even if angles of incidence and reflection are not mentioned.
  • After they have done the experiment, ask students to remember (from experimenting with a ripple tank) how waves behave on reflection.
  • Both particles and waves reflect in the same way as light.

This experiment was safety-tested in February 2006

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Refraction of particles and of waves

Refraction
Light, Sound and Waves

Refraction of particles and of waves

Practical Activity for 14-16

Demonstration

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

Apparatus and Materials

For the class

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.

Read our standard health & safety guidance


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

  1. 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.
  2. 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.
  3. 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

  • 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.
  • 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


  • 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-tested in February 2006

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Light – waves or particles?

Refraction
Light Sound and Waves

Light – waves or particles?

Teaching Guidance for 14-16

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

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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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Demonstration or class experiment?

Demonstration or class experiment?

Teaching Guidance for 11-14 14-16

Physics, more than any other science, can be demonstrated principle after principle by direct and simple experiments. In some cases, it is clear that an experiment should be done either as a demonstration or as a class experiment. But many experiments can be done in either way, each having advantages and disadvantages.

Demonstration experiments can clarify a physical principle or show some interesting application of a principle. Make sure that students in the back row, as well as the front row, can see and hear what is going on. The best demonstration experiments avoid unnecessary detail – students can see and understand the whole working arrangement.

Other reasons for demonstrating experiments include safety reasons, and limited apparatus. Demonstrations can also be used as a part of a revision session or when you want to draw quick comparisons, e.g. looking at the behaviour of water waves and comparing that with light or sound. In a short lesson, there may simply not be time for students to carry out their own investigations, after they have set up and dismantled ripple tanks.

Class experiments give students direct experience of physical phenomena. Just as important, they allow students to practise being scientists: discussing, developing hypotheses, designing experiments, predicting outcomes and returning to fresh hypotheses and more experiments. They develop their powers of observation, thinking and problem-solving. Active learning follows the adage ‘hear and forget, see and remember, do and understand’.

Because some students work more quickly than others do, it is a good idea to give students a series of questions to pursue. With a selection of extra equipment set out cafeteria style, students can then proceed at their own pace. That way all remain engaged and faster students accomplish more.

Through class experiments, students can learn:

  • how to devise experiments
  • how to work on their own
  • how to make mistakes
  • how to solve practical problems
  • how to enjoy success
  • and they learn a little theory too.

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Using wave simulations

Interference
Light Sound and Waves

Using wave simulations

Teaching Guidance for 14-16

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.

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