What the Activity is for

A visual and auditory demonstration of shifted frequencies.

Here you can listen to the Doppler effect as the relative velocity between you and a moving object is varied. For this you need a simple piece of apparatus, and a Doppler ball is recommended. There are two ways in which the Doppler effect is important:

• The source can be moving towards or away from you.
• The source can be spinning, so effectively moving.

What to Prepare

• a Doppler ball
• sound processing software

To make the Doppler ball take an old tennis ball and cut it in half. Now make a small square hole, to take a slide switch, in one of the halves (a slide switch is recommended because it is more robust). In the middle of one half make a small circular hole, perhaps with a cork borer. Mount a buzzer up against this hole. Now wire up a 9 volt battery to the buzzer and to the slide switch. Pack the ball with foam and stick the two halves together again. Check that operating the slide switch turns the buzzer on and off.

We'd recommend that you consider two variations to improve this basic set up.

One is to use a larger diameter ball, for the spinning experiment. Mount the buzzer on the equator. Larger is not necessarily better – choose one that you can spin relatively rapidly.

A second is to use a throw toy (shaped much like a rugby football, with fins to prevent tumbling when thrown) to prevent the ball rotating, when thrown. Mount the buzzer on the nose, but do remember to allow for its impact to be cushioned.

Safety note: The tennis ball is relatively soft, but obviously should not be thrown directly at students.

What Happens During this Activity

You need to practice. You should aim to throw the ball without it spinning. And you should be able to spin the ball on the table so that the buzzer spins round and round the axis of rotation (so along an equator).

Start by exploring expectations as the ball moves towards or away from the students – just along a line. Draw on their everyday experience in order to generate predictions. The change in pitch at quite reasonable speeds will be noticeable, but do make sure that the ball does not spin as you throw it. This demonstration will provide a clear experience of the Doppler effect.

More subtly, you can link measurements of Doppler spreading to a tabletop experiment. Here the ball is spun. So sometimes the source is travelling towards the students and sometimes it is travelling away from them. Therefore sometimes the pitch will be higher than if the ball was not spinning and sometimes it will be lower. So the emitted frequency is spread into a small range of frequencies as a result of the rotation. That is Doppler spreading. The faster the ball spins, the wider the range of frequencies. It is this that is used to measure the rate of rotation of distant astronomical objects.

You might like to use a microphone and sound processing software, showing frequencies, as the ball spins. It's much harder to make such measurements with the ball moving in a straight line, unless you're very skilful.

What the Activity is for

In this activity, students get to explore possibilities to find out why a refracted beam follows the path that it does. They use a physical model to carry out a mathematical experiment.

What to Prepare

• specialised cardboard rulers, measuring in nanoseconds
• outlines of refracting objects
• a sharpened pencil

What Happens During this Activity

Start by performing an impressive demonstration of refraction. It might also be a good idea to remind the students how beams refract by using rectangular glass blocks and beams of light.

In this context, the point of these activities is to create something to be explained. Discuss the outcomes of these experiments with students so that they recognise there is something that needs a good explanatory story. You'll need to build up their expectations so that they are not happy with a simple statement of the rules. Something about the angle of refraction, and the angle of incidence and one being larger than the other, should be the starting point, and not the endpoint.

So the puzzle is the following:

Teacher: Why does the beam follow the path that it does, as it passes from one medium to another?

Perhaps there is something rather special about the path that it does follow. You might suggest that the light follows the shortest path – that is the path with the shortest distance. It won't take much experimenting to show that this is false. You don't need any special rulers for this.

Another possibility is that the path that is followed is the one that takes the least time. More experiments will be required to establish whether this is true or false. These are mathematical experiments that we suggest you do physically. Some students in the class may complain that this is somewhat repetitive and boring. If so, you can always challenge them to produce a mathematical model (a computed model) that carries out the repetitive tasks for them. In any case, students need to systematically try out a number of paths and then to find the trip time for the light in travelling from source to detector. The paths will typically consist of two segments: one in air, and one in a different medium. Since the apparent speed of light in these two media is different, use two different rulers. One ruler converts distance to time for light in air; the other converts distance to time for light in the other medium. The trip time will be the sum of the times marked on the two rulers for the path that is explored.

Students should explore a number of paths and then look for something special about the path that seems most similar to the one they saw on the laboratory bench, concentrating on the transition through a single interface between the air and the medium. At this stage it may be useful to have a refracted beam set up in one corner of the laboratory to remind students what they are comparing their experiment with.

You might find it appropriate to encourage your students to plot a graph, showing the variation in trip time with some systematic variation in the path explored. One such possible variable that captures the path explored is the point at which the path passes from one medium to another. You might, a little light-heartedly, refer to this as a waypoint.

You might also like to arrange it so that students can easily compare the patterns they find, thereby averaging the experiment across the whole class.

You can easily extend this activity by using more complex shapes, which will inevitably involve three steps in calculating the trip time, as such shapes will involve two changes of medium. You might try a rectangular block and then a prism, as linked first steps.

What the Activity is for

In this activity, students get to explore why a reflected beam follows the path that it does. They use a physical model to carry out a mathematical experiment.

What to Prepare

• specialised cardboard rulers, measuring in nanoseconds
• outlines of reflecting objects
• a sharpened pencil

What Happens During this Activity

Start the activity by performing an impressive demonstration of reflection. It might also be a good idea to remind the students how beams reflect, by using mirrors and beams of light.

In this context, the point of these activities is to create something to be explained. Discuss the outcomes of these experiments with students so that they recognise there is something that needs a good explanatory story. You'll need to build up their expectations so that they are not happy with a simple statement of the rules. Something about the angle of reflection being equal to the angle of incidence should be the starting point and not the endpoint.

Teacher: Here's a puzzle – why does the beam follow the path that it does, as it reflects?

Perhaps there is something rather special about the path that it does follow. You might suggest that the light follows the shortest path – that is, the path with the shortest distance. Another possibility is that the path that is followed is the one that takes the least time. More experiments will be required to establish whether this is true or false. These are mathematical experiments that we suggest you do physically. Some students in the class may complain that this is somewhat repetitive and boring. If so, you can always challenge them to produce a mathematical model (a computed model) that carries out the repetitive tasks for them. In any case, students need to systematically try out a number of paths and then to find the trip time for the light in travelling from source to detector.

The path will typically consist of two segments, both in air, so you need a pair of identical rulers. Both rulers convert distance to time for light as it passes through air, so both will have identical gradations. Use one ruler for the incident path and one for the reflected path. The trip time will be the sum of the times marked on the two rulers for the path that is explored. Students should explore a number of paths and then look for something special about the path that seems most similar to the one they saw on the laboratory bench. At this stage it may be useful to have a reflected beam set up in one corner of the laboratory to remind students what they are comparing their experiment with. You might find it appropriate to encourage your students to plot a graph, showing the variation in trip time with some systematic variation in the path explored. One such possible variable, which captures the path explored, is the point at which the path hits the mirror. You might, a little light-heartedly, refer to this as a waypoint.

You might also like to arrange it so that students can easily compare the patterns they find, thereby averaging the experiment across the whole class.

You can easily extend this activity by posing questions about mirrors that are curved. Here it may be useful to have a single longer ruler that can be folded where the ruler hits the mirror.

What the Activity is for

Here we're looking at how stretching space predicts the change in recession velocity of galaxies. If you stretch a piece of elastic uniformly, the farther away any point is from the centre the faster it moves from that centre.

What to Prepare

• a length of elastic cord of low stiffness, with galaxies attached
• a metre ruler

Safety note: Take care to release the tension in the elastic cord carefully – choose the anchoring students wisely.

What Happens During this Activity

This is a demonstration, and will need building up. The explanation for cosmic redshift is that space itself stretches: that is, the space between galaxies not the space within galaxies. Here the galaxies are spread out along an elastic thread. It is best to anchor the galaxies at both of their extremities to avoid difficulties with space stretching through the galaxy.

Start with the elastic cord loose – just, but only just, pulled tight between two students, one at each end. Choose a galaxy to be home. Mark this galaxy in some way, perhaps with a Post-it note. Choose a pair of other galaxies to measure, one of which is further away than the other. Mark their positions. Now have the students at the ends move apart a short distance to simulate the evolution of the universe, as space expands. Mark the new positions of the chosen galaxies. From a change in position you can calculate their recession velocity – how fast they are moving away from the home galaxy. From their average position you can work out how far away they are.

The recession velocity increases as the distance from the home galaxy increases, as a simple result of the uniform stretching of space. The advantage of using elastic cord is that the space is is obviously uniformly stretched. You might measure the change in length of different parts of the cord to show that this is so. You might make multiple measurements across several galaxies to show that the recession velocity is proportional to the distance from the chosen or home galaxy.

The important final step is to point out that this is one direction only. You might like to make the link with the homogeneous make-up of space, leading to the idea that the recession velocities will vary uniformly in any direction.

What the Activity is for

A series of measurements showing exponential decay.

As you add a thickness of absorber, so the power in a beam decreases by a constant fraction. This leads to an exponential decay of the power in the beam.

Exponential decays (which are just constant fractional decays) will be met several times in radiation and perhaps be of particular interest when studying ionising radiation.

What to Prepare

• an optical light sensor
• a light source
• a microscope slide
• lengths of ghost, or mending tape

What Happens During this Activity

You will need to do some trial work, in order to match up the best distances with the sensor and light source that you choose.

This is best done as a student exploration. Skilful experimentation on the part of the student will give a truly beautiful exponential curve. You will have to guide the students as to how to set up the apparatus, so it's best not to call it an out and out investigation. The other reason for not labelling it an investigation is that the outcome is known – we are after a particular pattern here, and you should challenge all students to get it.

Students record and plot the power transmitted by increasing thicknesses of tape. The microscope slide is used to hold the tape. Emphasise adding one thickness of tape at a time. The question to explore: what happens when you add a single thickness of tape? That is, what's the pattern here? You could amplify the question either in terms of the power in the beam, the number of photons in the stream, or just in terms of the brightness. Decide on an approach well-matched to your class.

Students should be encouraged to explain in their own words the shape of the graph. A useful trick is to imagine that you have to tell your friend what you have found without being able to show them – how could you get them to draw the graph you have in front of you just by telling them about the shape (and not about the particular values that you happen to have got)?

How far you go in exploring the pattern here will depend on the aptitude and interest of the class, but we think it's useful to meet the exponential pattern as a variation with distance first because it's more obvious what is going on than for variations over time. You can step forwards as well as backwards through the pattern without extraneous difficulties, for example.

You can repeat the experiment as a demonstration, and use this to pull out the main points, by using microwave (3 cm) apparatus and textbooks for the incremental thicknesses. Choose your textbook wisely, so that 6 to 8 textbooks will decrease the power detected to a very low value. You might choose to use software to plot the values as you go. Indeed, you might use a data analysis program to explore the constant fractional nature of the decay. There's a lot here: choose the depth of treatment to suit the class.

What the Activity is for

Bats see somewhat differently from us – they see with sound. They also actively sense their environment, unlike humans. We see with reflected light but we don't produce that light. Bats build up a picture of their environment with sound that they produce.

This activity explores the source-medium-detector model, and encourages students to use it explicitly.

What to Prepare

• find or invent a piece of writing, or other media, on bats' abilities to echolocate eg

  Guardian website: Video footage shows young bats attempting to drink from a smooth metal surface in the same way they would from a pond or river

• a collection of bat sounds

What Happens During this Activity

Students read, listen to, or watch the expository media. They should translate what they comprehend into a diagram that explains, in terms of source, medium and detector, how the particular process occurs. You might set up the activity as compare and contrast, by having an environment in the laboratory that is sensed using the student's eyes.

You might also have available other pieces of apparatus which have been used for remote sensing using radiations: from the SPT: Earth in space topic; analogues of ultrasound scanning used elsewhere in your courses; or sonic rulers.

Students should end up with with a diagrammatic account of what the bat is doing in imaging its surroundings, perhaps comparing this with what humans do in constructing the scene they are looking at in the laboratory.

What the Activity is for

Diagnostic questions relating to sound and light waves:

• To encourage students to talk and think through their understandings of the fundamental properties of waves.
• To provide the teacher with formative assessment information about the students' understandings of waves.

What to Prepare

• printed copies of diagnostic questions (see below)

What Happens During this Activity

It would be a good idea to get the students to work in pairs on these questions, encouraging them to talk through their ideas with each other. Collect responses from all of the pairs and discuss in a whole class plenary.

Alternatively, the questions might be set for homework prior to the lesson, so that you have time to read through the responses.

Question 1: Which one of the following is correct?

• Light is a longitudinal wave that can travel through air.
• Light is a transverse wave that can travel through a vacuum.
• Sound is a longitudinal wave that can travel through a vacuum.
• Sound is a transverse wave that can travel through air.

Answer B.

Question 2: A laser pulse is sent out from Earth and after 4.5 × 10^-2 second, it is reflected back from a satellite in space. What is the distance between the Earth and the satellite?

• 4.5 × 10⁶ metre.
• 6.75 × 10⁶ metre.
• 9.0 × 10⁶ metre.
• 13.5 × 10⁶ metre.

Answer D. This is assuming that the laser pulse travels at the speed of light: 3 × 10⁸ metre inverse second.

Question 3: Which one of the following electromagnetic radiations can be used for cooking?

• Radio waves.
• Infra red radiation.
• X-rays.
• Gamma rays.

Answer B.

The final few questions and answers

Question 4: Two opera singers are practising. They are singing at the same loudness, but singer X sings a higher pitch note, and singer Y sings a lower pitch note. Which one of the following is true?

• The two amplitudes are the same, but the frequencies are different.
• The two frequencies are the same, but the amplitudes are different.
• The two frequencies are the same, and the amplitudes are also the same.
• The two frequencies are different, and the amplitudes are also different.

Answer A.

Question 5: A dust particle hovers in front of a silent loudspeaker. The loudspeaker is turned on and emits a loud musical note of constant frequency. How will the dust particle move?

• It will stay in the same position.
• It will move away from the speaker.
• It will move up and down at about the same position.
• It will move back and forth at about the same position.

Answer D.

Question 6: The frequency of the sound from the loudspeaker is increased but the loudness stays the same. What happens to the motion of the dust particle in front of the speaker?

• It stays at the same position.
• It moves back and forwards over a bigger distance.
• It moves back and forwards faster.
• It moves up and down over a bigger distance.
• It moves up and down faster.
• It moves away over a bigger distance.
• It moves away faster.

Answer C.

Question 7: The loudness of the sound is increased but the frequency stays the same. What happens to the motion of the dust particle?

• It stays at the same position.
• It moves back and forwards over a bigger distance.
• It moves back and forwards faster.
• It moves up and down over a bigger distance.
• It moves up and down faster.
• It moves away over a bigger distance.
• It moves away faster.

Answer B.