What the Activity is for

Seeing beams of light.

This is part of a suggested sequence:

A repertoire to develop the idea of seeing

Pupils will very often agree that to be able to see, light must enter the eye. However, it is possible to set up situations where this apparently firmly-fixed belief can be productively challenged, and thereby strengthened. This is the aim for this demonstration.

What to Prepare

• a source of fine dust – board duster or talc
• a low power laser that provides an intense, fine beam of light

Safety note: Take care to ensure that light from the laser cannot pass directly into anyone's eyes. A class 2 laser should be used, although a laser pointer can be used if under the careful control of the teacher. You should be aware, however, that some laser pointers are incorrectly assigned a relatively low power rating and are potentially more dangerous than they might appear.

Even if a laser beam does enter the eye, the blink-aversion response is sufficiently fast to avoid any damage to the retina except with the most powerful lasers.

What Happens During this Activity

Set up the laser so that, when switched on, the beam travels across the front of the room, landing on a white screen so that the red spot is clearly visible.

Start by describing the set-up to the pupils, showing the laser and pointing to the screen across the room at which the beam is directed. For the sake of this demonstration, all that the pupils need to know is that the laser provides an intense, or very powerful, beam of light.

Explain that you are going to put the room lights out in a moment and ask the pupils to predict what they will see when the lights are out (with a good black-out) and the laser is switched on. This is where the pupils' ideas about seeing are challenged.

Challenging predictions

Former opinion: You'll see the beam cutting through the darkness.

Latter opinion: You'll just see a spot on the screen where the beam hits.

Experience has shown that the former opinion is very common among pupils in the lower secondary school age range. Make sure to bring it out into the open, so that it gets effectively challenged. It's not enough to simply show the process and assume the learning will be automatic.

Now switch on the laser (with appropriate theatrical build-up). Nothing seems to happen. A red spot appears on the screen at the side of the room, but the laser beam itself can't be seen.

The big question is: Why can't we see the laser beam? (Because none of the laser light is entering our eyes). This can lead to an interesting discussion of situations where it is possible to see beams of light cutting through the air. Pupils may well refer to laser shows at pop concerts, the projection lights in cinemas, car headlamps on a foggy night. In each of these cases there is something which will scatter the light into our eyes.

An interesting follow-up question is: How come we can see the red spot on the screen? (Here laser light is being reflected in all directions from the wall and some travels to our eyes).

Use chalk dust from the board rubber (if you still use one) or a fine powder such as talc, or alternatively a fine water spray to scatter the beam. The room light is off and the effect is stunning as the beam becomes visible as light is scattered away from it.

What the Activity is for

This activity focuses on the idea that anything producing a sound must be vibrating. We think it best to start off with sources that involve obvious vibrations and then look at more subtle movements later.

What to Prepare

You will need a collection of things that vibrate along with the means of showing that they do vibrate. We suggest:

• a large panel of hardboard (approx: 1 metre by 1 metre) popularised as the wobble board
• a tuning fork and a bowl of water, together with a table tennis ball suspended on a long piece of fishing line
• a large loudspeaker, a signal generator and a collection of ball bearings or dried peas
• a strobe light
• a laser and a small piece of plastic mirror

To take it further:

a small wind-up music box and access to a table to use as a sounding board a small loudspeaker (50 millimetre diameter) and access to a pane of glass

Safety note: Signal generators driving loudspeakers have been known to produce unpleasant feelings, even nausea, in susceptible individuals at a variety of frequencies from as low as 4–15 hertz to above 20 kilohertz.

Safety note: Strobe light: photosensitive epilepsy is very rare but anyone suffering from it must not be exposed to flickering light. The same applies to those prone to migraine attacks.

Safety note: Lasers approved for school use should be used. Ensure that the beam cannot enter anyone's eye either directly or by reflection. A laser pointer can be used under the careful control of a teacher but be aware that some are marked with an incorrect power rating and so are more hazardous than they might appear.

What Happens During this Activity

Making sources

Start with the wobble board – just wobble it to and fro slowly in an exaggerated action. As you increase the rate of wobbling, start to talk about the vibrations of the board, connecting this use of vibration with the to and fro action. Ask what the board is doing to the air particles next to it. Slow the action right down so you can emphasise the push on the particles, and their bouncing back when this push is no longer acting. Emphasise that the vibrating board sets huge numbers of particles into this to and fro motion. You might also try to count the vibrations in a set interval of time, setting the scene for the introduction of the concept of frequency.

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Introduce the tuning fork as something which makes a sound, but which we cannot see moving. (In fact our eyes cannot react fast enough to catch it moving, added to which it is not moving very far.) You can show that the prongs of the tuning fork are moving by dipping the end in a beaker of water. Try drawing the pupils in close as you demonstrate this on the pretext of trying to see the very small vibrations. The pupils will be wetted by the water particles as they are displaced by the to and fro action of the fork.

Alternatives

An alternative is to suspend a table tennis ball on a long piece of fishing line beside the vibrating fork, so that the fork collides with it at the extremity of its vibration.

A further way of seeing the vibrations is to polish up one end of the tuning fork (right at the end) and then bounce a laser beam off the outside of the end at a glancing angle (a laser pointer is enough), so that the reflected spot ends up on the wall a long way off.

The laser beam set-up amplifies the vibration of the fork. If you cannot polish up the end sufficiently, attach a small piece of plastic mirror to the end with a piece of Blu-tack. You might want to model what you are doing, perhaps by monitoring small movements on a large object such as the laboratory door.

To make lots of particles vibrate (as you did with the wobble board), just place the stem of the tuning fork on a wooden table and listen out as the table is set vibrating. A nice alternative is to have a music box playing, first with it held in the air and then with it resting on the table.

A further activity at this stage involves setting up a circus of vibrating objects (including a range of musical instruments) and getting pupils to spot the vibration that is acting as the source of the sound.

With the loudspeaker, you could use a strobe light flashing slightly more or less often than the number of loudspeaker vibrations a second to give snapshots. We think, however, that it would be better to keep things simple. Place the loudspeaker so that it faces upwards and then put (small) ball bearings or (dried) peas on the surface. With the loudspeaker connected to the signal generator, the peas or ball bearings jump about as the speaker cone vibrates.

Two extensions to discuss:

• Rooms can be bugged by bouncing laser beams from the windows (health and safety is not so much of a concern for spies, but we don't suggest you try this), detecting any to and fro motion of the window pane. What would create such a motion? This is the stuff of spy stories.
• Small loudspeakers are available that can be clamped to window panes. The loudspeaker sets the whole window pane vibrating to get lots of particles moving. This produces a much louder sound.

What the Activity is for

The aim of this activity is to make the link between the loudness of a sound and the magnitude of the vibration that produces it. The louder the sound, the bigger the vibration (the bigger the amplitude of vibration). The decibel scale is then introduced as a standard scale for measuring loudness.

What to Prepare

• a wobble board
• a selection of pre-recorded sounds, or the means for making them. You will need to be able to alter the loudness of the sound.
• a laser beam, reflecting from a strip of plastic mirror stuck to a vibrating loudspeaker. Make sure the reflected beam has a safe path to a distant wall.
• an oscilloscope, or computer-based oscilloscope, connected to a microphone, with the time-base switched off.
• a decibel meter, preferably linked to a large display.

Safety note: Lasers approved for school use should be used. Ensure that the beam cannot enter anyone's eye either directly or by reflection. A laser pointer can be used under the careful control of a teacher but be aware that some are marked with an incorrect power rating and so are more hazardous than they might appear.

What Happens During this Activity

Start with the wobble board, keeping the frequency the same, but driving the board into larger vibrations. Make the connection between this extra movement and a louder sound. You may need to practice this.

Now use the loudspeaker and laser beam to make the link between the amount of movement of the loudspeaker cone (shown by the movement of the laser dot on the wall) and the loudness of the sound. As you turn up the loudness control, the laser beam will move back and forth over a greater distance on the wall.

You can then use the microphone and oscilloscope to show the increase in amplitude on the screen as the sound gets louder. Play louder and softer sounds into the scope and watch the size of the vibrations increase and decrease.

Finally, introduce the decibel scale by demonstrating how a large decibel meter can be used to measure the loudness of the sounds. It is a good idea to demonstrate the link to decrease in loudness as you move further from the source. Also make connections between your readings taken in the lab and common values between the 0 dB threshold of hearing and the 140 dB threshold of pain.