Progressive Wave
Light, Sound and Waves

Variety of waves

for 14-16

Introducing students to waves of many types, and the language used to describe them.

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Introducing waves

Progressive Wave
Light, Sound and Waves

Introducing waves

Practical Activity for 14-16

Demonstration

The point of these questions is to stimulate curiosity and to demonstrate that waves have some interesting properties. At an introductory stage, you must leave many questions unanswered.

Apparatus and Materials

  • Sound sources, such as tin whistles, guitar, radio
  • Small pieces of closely-woven fabric for each student
  • Lamp (e.g compact light source, 12V
  • Low voltage power supplies, 2 (compact light source will need 8A low voltage power unit)
  • Radiant heater, low voltage

Health & Safety and Technical Notes

Do not use a mains-powered electric fire with a bare element. Use either a low-voltage infra-red source or (better) a mains-powered one with the element(s) inside silica tubes.

Read our standard health & safety guidance


Procedure

Run through a series of questions and demonstrations about various wave phenomena, for example:

Sound: echo and speed

  • Say: "When you talk, sound waves carry your message through the air to other people. Clap your hands and listen for the echo."
  • Ask: "How do sounds travel so fast? Can sound waves in air travel faster than the wind?"
  • Ask: "How do you know that sound carries energy?"

Sound: beats

  • Play two sources of sound of slightly different frequencies, using whistles, small organ pipes, monochords, or tuning forks.
  • Say: "The throbbing sound is called 'beats'"
  • Ask: "How do two sounds create these beats?"

Waves at sea

  • Ask: "How do waves travel across an ocean from a remote storm?"
  • Ask: "How do you know that ocean waves carry energy?"
  • Ask: "Have you noticed that wave crests get closer together as they roll up a beach? That they change direction as they round a head, or enter narrow gates of a harbour?"

Radio

  • Say: "The signals of radio and television come to us as waves."
  • Ask: "What carries them?"
  • Ask: "When you hear a time signal on the radio (or catch a programme on TV), do the signals arrive noticeably later if you are far away from the broadcasting station?"
  • Say: "Those signals bring in a tiny stream of energy that triggers a lot more energy from the mains or batteries."

Light

  • Give each student a small piece of closely-woven cloth. Get students to look at the lamp through the cloth. Also, suggest looking at a distant street lamp at night through an umbrella.
  • Ask: "Why do you see an extra pattern of bright spots?"
  • For comparison, set up a funnel over the table and put fine dry sand in it. Hold a fine-mesh strainer in the stream of sand."
  • Ask: "does the sand make an extra pattern of heaps when it goes through a mesh or just a single pile?"
  • Ask: "What makes the difference between light and sand patterns?"

Radiation, visible and invisible: Energy

  • We call the colours of visible light, infra-red and ultra-violet, and so on ‘radiation’. They all carry energy.
  • Ask: "What do you feel when you hold your hand near a glowing fire or an electric heater?"
  • Ask: "What happens if someone walks between you and the heater? Can you still feel its warmth?"

Teaching Notes

  • The aim of this introductory discussion is to whet students' appetite for learning more about waves. Waves are all around us and behave in interesting ways. You could answer students' questions with further questions.
  • The explanations given below are for teachers only. Students' understanding should develop gradually, later, as they try further experiments.
  • Sound echo and speed: Sound can travel faster then the wind. The waves are carried by collisions between molecules in air transferring movement energy. A sound wave is made up of pressures that are alternately higher (at compressions) and lower (at rarefactions). There are many ways that you can tell sound carries energy, e.g. it pushes eardrums in-and-out.
  • Sound beats: Waves are unique in their ability to simultaneously pass through any point without permanently affecting each other. But the effect at a point is a superposition of the effects of the waves that are passing. This is called interference. The way that two sounds of slightly different frequency interfere produces beats. Musicians use this effect in tuning their instruments.
  • Waves at sea: Sea waves are carried in a different way than sound waves. There is an attractive force between water molecules - when one molecule moves, it makes its neighbour move too. You can see that waves carry energy because waves move stones on a beach, boats or people in the water. The change in spacing and direction of wave peaks along a shore shows that the wave speed decreases as the water gets shallower.
  • Radio: Radio waves are a form of electromagnetic wave. There is no noticeable time delay because the speed of these waves is the speed of light, extremely fast.
  • Light: When they look through the cloth, students will see a diffraction pattern. Only waves, and not particles, make diffraction patterns. This is evidence that light is a wave.
  • Radiation, visible and invisible: To be more precise, this is called electromagnetic radiation. A radiant heater produces infrared light. It behaves just like visible light, and opaque objects (e.g. the person who walks between you and the heater) will cause shadows.

This experiment was safety-tested in February 2007

Up next

Red-hot heater and curved mirror

Progressive Wave
Light, Sound and Waves

Red-hot heater and curved mirrors

Practical Activity for 14-16

Demonstration

This sequence of demonstration experiments could be used as part of an introduction to waves, in which case students' questions could be answered with further questions.

It could also be used to demonstrate that infra-red radiation reflects in exactly the same way as visible light, or to get a qualitative feeling for its speed.

Apparatus and Materials

  • Radiant heater, 300-watt
  • Parabolic mirrors, metal-surfaced, 1 pair

Health & Safety and Technical Notes

The old types of radiant heater are not considered sufficiently safe for use in schools. A 300-W safe version is available (see apparatus).

Beware of burns: tell students to stop as soon as they feel anything.

Read our standard health & safety guidance


Procedure

  1. Set up the mirrors facing each other, 2 or 3 metres apart.
  2. Place the heater at the focus of one of them—using the visible red radiation as a guide.
  3. Find the image of the heater formed by the second mirror.

Teaching Notes

  • Let students place a hand at the focus of the second mirror, by turns. Avoid telling them what to expect but suggest placing the hand with palm towards the other mirror.
  • Say: "Some energy seems to go across from one mirror to the other."
  • Ask: "How could energy get across there, so far away?"
  • Hold a large sheet of cardboard or plywood as an obstruction just beyond the heater.
  • Ask students to try again with a hand at the focus of the second mirror.
  • Whip the board away quickly.
  • Ask: "What do you feel? Does the supply of energy start again almost at once or is there a delay — as it would if air currents carried the energy?"
  • With a student’s hand at the focus of the second mirror, suddenly obstruct the radiation again.
  • Ask: "How quickly was the warmth cut off? How fast does the radiation travel?"

This experiment was safety-tested in September 2004

Up next

Beyond the visible spectrum

The Electromagnetic Spectrum
Light, Sound and Waves

Beyond the visible spectrum

Practical Activity for 14-16

Demonstration

This experiment can be used when introducing the electromagnetic spectrum. It shows that there is a form of radiation beyond the visible.

Apparatus and Materials

  • Prism, high-dispersion
  • Light source, compact
  • Positive lens, large
  • Screen, white
  • Infra-red detector, metered output
  • Infra-red detector, metered output

Health & Safety and Technical Notes

Read our standard health & safety guidance


For an infra-red detector, make a potential divider using a photo-diode in series with a 100kΩ resistor, and a 5 V supply across the pair. Connecting a digital voltmeter across the resistor will give the required metered output.

Compact light source: 100 W at 12 volts

The power supply should supply 8 amps.

Procedure

  1. Set up the compact light source. The lamp filament should be small enough that no slit is needed.
  2. Place the lens about 20 cm from the lamp. (If the lens is plano-convex, its plane face should be towards the lamp.) Move it to make an image of the filament on a white screen, 2 or 3 metres away.
  3. Place the prism just beyond the lens and move the screen round to catch the spectrum at the same distance from the lens as before but in the new direction. The spectrum will be pure enough for this demonstration if the prism is turned to minimum deviation. To make the spectrum longer, twist the screen to catch it obliquely.
  4. Move the detector across the spectrum near the screen and observe the output readings.

Teaching Notes

The radiation emitted by a hot filament has its maximum not in the visible spectrum, but beyond the red (in the 'infra-red' part of the spectrum). Light is just one (small) part of a family of radiations called the electromagnetic spectrum.

This experiment was safety-tested in February 2006

Up next

Transverse waves along a rope

Progressive Wave
Light, Sound and Waves

Transverse waves along a rope

Practical Activity for 14-16

Class practical

Introduce the concept of a transverse wave by drawing attention to a wave pulse as a displacement at right angles to the direction of travel along a rope.

Apparatus and Materials

  • For each student pair
  • Length of flexible rope (the longer the rope the better, minimum length 3 metres). The rope should be a massive, loose rope - not a stiff one.

Health & Safety and Technical Notes

Read our standard health & safety guidance


A lot of space is needed for students to demonstrate waves along ropes. If the whole class is to do this at one time, arrange to use a long, wide corridor or the school hall.

Procedure

  1. Students work in pairs, each holding one end of the rope in mid-air. One tries to hold the rope still. The other uses a hand to jerk one end of the rope up and quickly down, stopped on the wrist of his/her other hand.
  2. Alternatively, this could be done on a bench or on the floor.
  3. If the rope is held in mid-air, pulses are likely to reflect back and forth several times.

Teaching Notes

  • Draw students’ attention to the way that the pulse diminishes in size as it moves along the rope. You may also want them to observe the inversion of the pulse after each reflection.
  • Challenge the student holding the far end of the rope not to let their hand move. That they cannot do so always surprises and intrigues them - and makes it clear that energy is being delivered.
  • Students may produce both continuous waves and standing waves without further instruction.
  • To produce standing waves: mark off the rope into equal segments, such as fifths. Holding the ends of the rope tightly, each person makes a loose ring with a finger and thumb round the rope at the nearest marked point. One person then moves the rope up and down, adjusting the frequency until the 5-loop motion builds up. Different resonances can be produced, of course, by changing the frequency of the tension. This impedance matching will produce an effective standing wave.

This experiment was safety-tested in February 2006

  • This video shows how to model transverse waves using a simple to construct wave machine:

Up next

Pulses and continuous waves with a Slinky spring

Progressive Wave
Light, Sound and Waves

Pulses and continuous waves with a Slinky spring

Practical Activity for 14-16

Demonstration

Use this experiment when introducing transverse and longitudinal waves. Later it might also be used when introducing standing waves or factors that affect wave speed through a medium.

Apparatus and Materials

For the demonstration, or for each pair of students

  • Slinky spring
  • Rubber tubing

Health & Safety and Technical Notes

Read our standard health & safety guidance


The Slinky spring should be at least 10 cm long when closed up. It is useful to tie a bright ribbon marker on one loop of the spring, so that students can watch how a single loop moves when a pulse passes it.

The length of rubber tubing should be at least 5 metres long, diameter at least 8 mm.

Procedure

Pulses

  1. Hold the rubber tubing on the floor under slight tension. Give one end a sharp flick horizontally. This is most easily done by holding a hand against the ankle and then jerking the tubing sideways and back to the foot again.
  2. Try different tensions and slower pulses. (Pulses of a different shape can be produced by having a stop such as a chair leg to limit the motion of the tube.)
  3. Repeat the same demonstration with a Slinky.

Continuous waves

  1. Lay the rubber tube or Slinky on the floor or on a bench.
  2. Fix one end and make the other end oscillate transversely by hand, with a small amplitude and a frequency of about 5 cycles per second.
  3. Impulses at regular time intervals will produce a continuous travelling wave. This is usually clearer with the rubber tubing.
  4. With a Slinky it is also possible to show travelling longitudinal waves by oscillating the end of the Slinky backward and forward.

Teaching Notes

  • If your main object with the Slinky is to see how pulses and continuous waves travel, avoid distractions such as producing standing waves or making the Slinky walk down stairs. Pulses of different shapes will be seen if you demonstrate both a rubber tube and a slinky spring.
  • You can produce both longitudinal and transverse wave pulses on a slinky spring, to contrast and compare them.
  • In transverse waves the particles oscillate at right angles to the direction of travel of the wave. In longitudinal waves the particles oscillate along the direction of travel of the wave.
  • One difficulty can arise the graphical representation of a longitudinal wave. Often it is represented in the same way as a transverse wave, and this can be misleading. Remember that the particle displacement, at any particular point in the medium, is really in the direction of travel of the wave. The diagrams below explain what the displacement - distance graph means.
  • It is interesting to double and triple the length of the springs and to note that the pulse takes the same time to travel down it. The increased tension needed to extend the spring has resulted in an increase in velocity. For example, double the tension by doubling the length of spring, which halves the mass per unit length. This means the tension/mass per unit length will be four times as great. The pulse must be travelling twice as fast if the time of travel has remained the same. Thus velocity2 = constant x tension / mass per unit length.

This experiment was safety-checked in February 2006

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Water waves seen in section

Progressive Wave
Light, Sound and Waves

Water waves seen in section

Practical Activity for 14-16

Demonstration

Another way of demonstrating the transverse motion of particles in a water wave.

Apparatus and Materials

  • Tank, large rectanglar transparent
  • Paddle (wood block with handle)
  • Sawdust, fine, small amount
  • Paraffin, coloured

Health & Safety and Technical Notes

Fine sawdust in the air is hazardous. Provide only a small quantity (e.g. 100 ml) in a small beaker or jar.

Read our standard health & safety guidance


A long tank is preferable to a short one, so that the initial outgoing waves are not immediately affected by the waves reflected from the end. The ideal is depth 15 cm, breadth 7 cm, length 1 m.

The paraffin layer on top of the water layer has to be added carefully so that air bubbles are not trapped between the two layers. Attach a funnel to some rubber tubing and place the other end of the tubing on the water surface. Pour paraffin gently into the funnel to produce a transparent layer on top of the water.

Cleaning the tank is very messy but a strong detergent will do the job. Decant the paraffin off the top and store it until the next use.

Procedure

  1. Fill the glass tank half full with water and place it so that the students can see the water line face on and any waves passing along it in cross-section.
  2. Generate waves at one end by moving the hand or a block of wood up and down in the water or, better, sweeping it back and forth as a paddle.
  3. The students watch the motion.
  4. If you mix some sawdust in the water those watching at close range may see the path of the particles of the medium when the water waves travel along. However, that motion is too fast to see easily so try step 5.
  5. Fill the tank one-third full of water and add paraffin (preferably coloured) above that until the tank is two-thirds full. Generate transverse waves at the interface, keeping the paddle immersed.

Teaching Notes

  • Before looking down on waves (ripples) in ripple tanks, it is useful to look at water waves in section in a long plastic tank.
  • To produce waves at the interface of the water and paraffin, oscillate the paddle in the water only. Try to avoid producing waves at the air/paraffin interface. With slower wave velocities it may be possible, by adding fine sawdust, to show that the motion of the water ‘particles’ close to the surface moving in circular paths and those in deeper water moving in elliptical paths.

This experiment was safety-tested in February 2006

Up next

Waves along a line of students

Progressive Wave
Light, Sound and Waves

Waves along a line of students

Practical Activity for 14-16

Class practical

Introduce transverse and longitudinal waves with a kinaesthetic experience. This can help students to understand and remember what each of these wave types are.

Apparatus and Materials

Students.

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Students stand side to side and link arms in a line. Send gentle transverse waves and pulses down the line.
  2. For longitudinal waves and pulses, students all turn right (or left) and place their hands on the shoulders of the student in front, with elbows kept bent.

Teaching Notes

  • To carry out these experiments successfully, you will need class discipline almost at the military parade ground level. They do illustrate clearly, however, the motion of particles in a medium that constitutes a passing wave.
  • With students in the second arrangement, you could ask the students to imagine what happens when a medium is strained beyond its elastic limit.
  • From the rear of the line, imagine that the end student is given a good shove to send a strong pulse down the line. Think what would happen next. When the students have figuratively picked themselves up, discuss the difference between this pulse and all the others so far: that the particles did not, in this case, return to their original places.

This experiment was safety-tested in February 2006

  •  A video showing another way to model transverse waves is using a wave machine (cheap and simple to construct):

Up next

Infra-red and ultraviolet radiation

Progressive Wave
Light, Sound and Waves

Infra-red and ultraviolet radiation

Practical Activity for 14-16

Demonstration

This experiment shows the overlap of infra-red radiation, visible light and ultraviolet radiation, all produced by the same source.

Apparatus and Materials

  • Parallel beam projector
  • Low voltage power supply, variable
  • High dispersion prism
  • Phototransistor
  • Cell holder with one U2 cell (1.5 V)
  • milliammeter (1 mA)
  • Infra-red and ultra-violet filters
  • Printing paper (P153) developer, fixer OR fluorescent paper (green)
  • Screen of non-fluorescent white paper and support
  • 4mm leads

Health & Safety and Technical Notes

Read our standard health & safety guidance


The parallel beam projector and prism are used, as shown below, to cast a spectrum onto a screen made of a piece of board about 0.3 x 0.2 m to which is pinned a sheet of non-fluorescent white paper set up about 0.5 m from the projector. As usual, adjust the lamp until its image would be in focus on the screen, and rotate the prism to the position of minimum deviation. The white paper, which could be blotting paper, can be tested for lack of fluorescence with an ultra-violet lamp and the ultra-violet filter, in a dark room.

For demonstration purposes, the lamp may be overrun by up to 30 per cent. It should have a linear filament.

Procedure

    Infra-red
  1. Connect the phototransistor to the U2 cell and milliammeter in series, and put it just in front of the screen. If the projector is rotated to sweep the spectrum across the phototransistor, a peak response will be found beyond the visible red region.
  2. With the transistor in the region beyond the peak, the effect of the filters can be shown.
  3. Ultra-violet: Do either 3 or 4. Step 4 is quicker, but shows less of the ultra-violet.
  4. Pin a strip of daylight printing paper to the screen and expose it to the spectrum for several seconds, marking the limit of the visible blue-violet with a soft pencil. Subdued incandescent room lights may be left on. Develop the paper in front of the class, when it will be seen that the paper is blackened well beyond the visible region. The dyes in the paper make it insensitive to parts of the visible spectrum.
  5. Pin the strip of fluorescent paper so that the lower half of the spectrum falls on it, the upper half still falling on the white paper. In a darkened room, some fluorescence can be seen beyond the visible if the lamp is overrun. Much of the fluorescence is in the visible blue-violet, but the difference is shown up by use of the ultra-violet filter to remove much of the visible region.

Teaching Notes

  • This experiment lends some support to the family view of electromagnetic radiation is the degree of overlap in properties shown by different parts of the spectrum.
  • For example, long wave radiation from a hot object can warm things up, and so can the radiation with wavelength of the order of centimetres or millimetres used in modern radar, and indeed also in microwave ovens. But these radio waves in turn share many properties with other radio waves of much longer wavelength.
  • A phototransistor will detect radiation in a spectrum from a lamp across the visible and well beyond the red, in the infra-red region where the most noticeable property is the warming up of an object held in the radiation.
  • Photographic paper is affected over much of the visible spectrum, but also well beyond the blue, in the ultra-violet region where another noticeable effect is the fluorescing of certain paints.

This experiment has yet to undergo a health and safety check.

Up next

Stroboscope: introduction

Progressive Wave
Light, Sound and Waves

Stroboscope: introduction

Practical Activity for 14-16

Class practical

The hand-held stroboscope is a simple device that can be used in several very useful ways.

Apparatus and Materials

Health & Safety and Technical Notes

If a fractional-horsepower motor (Fracmo) is used in this activity, take care to connect up the field (stator) coils and the armature (rotor) coils before connecting both to the power supply. These connections should not be changed while the motor is running.

The leads used to connect the motor should be fitted with 4 mm plugs having sprung shrouds (see the warning label enclosed with them).

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.

Teachers in maintained schools should check to see whether or not their Local Education Authority has given specific guidance on this matter.

Read our standard health & safety guidance


The rotating disc is black, painted with a white arrow.

The compact light source has an 8A low voltage power unit.

Retort stand and boss are needed for both lamp and stroboscope. Xenon stroboscope is needed for one of the experiments.

Procedure

To explain the principle of measuring a frequency with a stroboscope

  1. Start by swinging your arm slowly round in a large vertical circle.
  2. Ask students to close their eyes and to open them briefly each time you say ‘now’, once each revolution. Students will see your arm each time in the same position.
  3. Then say ‘now’ once every two revolutions so they see the same thing but less often.
  4. Finally, say ‘now’ once every half revolution, so they see your arm in two positions.
  5. You can find the frequency of rotation from the maximum number of rotations of the stroboscope per second, which show your arm frozen in just one position; more positions and the stroboscope is turning too quickly.
  6. Summarize: the correct speed of rotation is the highest speed that ‘stops’ the object. The frequency of rotation is then the number of rotations of the stroboscope per second multiplied by the number of slits in the stroboscope. If the flash frequency is such that n stationary images are seen then the speed of rotation being measured will be N = (speed of flashes per minute)/n. (Thank you to Manoj Chouksey who suggested we include this sentence.)

Students measure a frequency

  1. Use a motor to drive a black disc painted with a white arrow, at 25-30 revolutions per second.
  2. Students should be able to rotate their stroboscopes at the correct speed. The number of slits passing the eye per second (12 glimpses per rotation multiplied by the average number of rotations per second) is equal to the number of revolutions per second of the disc.

Another method – strobing with light

  1. Black out the room and use a lamp with a very bright small filament to illuminate the motor-driven disc.
  2. Place a converging lens to form a real image of the filament of the lamp on the stroboscope disc.
  3. Now rotate the stroboscope disc, so that light flashes on the rotating disc at a rate that ‘stops’ the motion of the arrow.
  4. This shows an alternative method to rotating the stroboscope in front of the eye.

Students measure mains frequency using a xenon stroboscope

  1. Set up a large neon lamp on AC mains. Do this experiment in daylight so that the lamp is visible even when the neon glow is not.
  2. Gradually increase the rate of flashing of the xenon stroboscope, until the lamp seems always to be ‘on’. This will be twice mains frequency, i.e. 100 per second, as the lamp lights with each voltage pulse.

Teaching Notes

  • If a student cannot see the stopped motion, you can help her/him by working the stroboscope, looking through one side of it while s/he looks through the other.
  • Hand stroboscopes are difficult to turn at high and low speeds. To show the effect of turning the stroboscope at half speed, and twice and three times the correct speed, you will need to run the motor at different speeds. Furthermore, at low speeds the white arrow becomes very spread out and indistinct, particularly at the edge of the disc where it is travelling fast.
  • It’s easy to rotate the stroboscope at the wrong speed when measurements are being taken:
    • Disc at 15 revolutions per second
  • It’s difficult to turn the 12-slit stroboscope slowly enough to see a single stationary arrow. But if the stroboscope is speeded up until it is twice as fast, three times as fast, and even four times as fast, a stationary pattern is seen.
    • Disc at 50 revolutions per second
  • It’s possible to ‘stop’ the motion of the arrow by turning the stroboscope at the correct speed, at half this speed, and at one-third of this speed. It’s not, however, possible to check that the highest of these three speeds is the correct speed – it is too difficult to rotate the stroboscope fast enough to get the ‘twice-as-fast’ pattern.
  • Some examples for discussion or for student investigation:
    • Wagon wheels on the cinema screen.
    • A cinema screen itself through a stroboscope.
    • Fluorescent lighting or street lighting through a stroboscope (calculating its frequency would require a 24-slit hand stroboscope.)
    • Hi-fi turntables (for old-fashioned vinyl recordings) turn relatively slowly. Some models have a large number of radial white bars marked near the circumference. When the speed is correct, each bar moves ahead one place for each flash of the mains lighting (100 flashes/second).
    • A fan with several blades can be ‘stopped’ with various speeds of the stroboscope. If one blade has a white marker, however, it becomes apparent that many of these speeds do not give the actual speed of the fan.
    • Observe the back wheel of an inverted bicycle through the stroboscope. Many speeds of the stroboscope appear to stop the wheel if the spokes look alike.

This experiment was safety-checked in August 2006

Up next

Why experiment with waves?

Standing Wave
Light, Sound and Waves

Why experiment with waves?

Teaching Guidance for 14-16

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.

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.

Up next

Some ideas for home experiments: waves

Standing Wave
Light Sound and Waves

Some ideas for home experiments: waves

Practical Activity for 14-16

Students generally enjoy devising their own investigations, using materials readily found at home. From time to time, this can be a good alternative to standard homework, especially when it leads to writing short reports or oral presentations. Home experiments give valuable practise with practical problem-solving and with conceptual thinking.

1 Standing waves in a rectangular tank

It is easy to excite water into a slopping mode, which is why it is hard to carry pans of water. Ask students what they can say about the Q (quality factor) of this system. It may be possible to excite other modes of oscillation using the plunger, for example, by moving it up and down in the middle of a large tank of water, and in other positions.

2 Standing waves under a running tap

Holding a knife-blade about 2 cm below a smoothly running tap, look for standing waves in the water flow. How does the pattern depend on the speed of water flow? On the separation of knife and tap?

3 Step waves under a running tap

Water flowing down from a tap onto a flat surface, such as the under-surface of a baking tray, shows a strange discontinuity: as the water flows away from the impact point, there is a step-like increase in its depth, at a distance r all round the impact point. Find out what factors affect r and try to explain this phenomenon.

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