Waves

1. Waves move energy, not a whole material

A wave is a disturbance that transfers energy. It can also carry information, such as speech in a phone call.

Evidence: a cork bobs as ripples pass without travelling across the tank with them. A loudspeaker makes nearby air vibrate; a listener can hear the sound without a steady stream of air arriving from the speaker.

2. Four measurements: do not mix them up

3. Calculate wave speed and period

1. Write the equation.
2. Convert lengths to metres and times to seconds.
3. Substitute the values, calculate and include the correct unit.

A wave’s speed tells you how fast its energy moves. Frequency tells you how often waves arrive. They are different quantities.

4. Required practical 20: ripples on water

1. Set up a shallow ripple tank with a straight vibrating dipper. Use a lamp and screen or a recorded top view to see the wavefronts.
2. Place a ruler beside the projected image or use a calibrated image. Allow for magnification: the screen image may be larger than the actual tank pattern.
3. Measure the distance across several wavelengths; divide by the number of crest-to-crest gaps.
4. Find frequency by counting complete waves in a timed recording, or use a checked dipper frequency.
5. Calculate speed using v = fλ. Repeat readings and find a mean.

Improve the measurement: use several gaps, keep water depth constant, reduce reflections from the tank edges and view from directly above. Keep water away from electrical connections.

5. Required practical 20: waves in a solid

1. Connect a vibration generator to a stretched string. Use a signal generator to set the frequency.
2. Keep string type and tension constant. Adjust the frequency until there is a clear stationary pattern.
3. Measure the length of several loops with a ruler. One loop is half a wavelength; two loops are one wavelength.
4. Use the generator frequency and the calculated wavelength in v = fλ.
5. Repeat. Judge which apparatus gives the clearest pattern and most reliable measurements.

Standing waves are used here as a measurement technique; you do not need a separate theory of standing waves. Secure the pulley and keep feet clear of the suspended mass.

6. How could you measure the speed of sound?

1. Place two microphones a measured distance apart, in line with the sound source.
2. Make a short sound. Use the same timer or data logger to record the delay between the microphones.
3. Calculate speed = separation ÷ delay. Repeat and use the mean.

An alternative: time an echo from a distant wall. The sound travels to the wall and back, so use twice the wall distance. Time several echoes or use recording software to reduce timing uncertainty.

7. Meet the electromagnetic family

• All electromagnetic waves are transverse.
• They transfer energy from a source to an absorber.
• They can travel through a vacuum. They all have the same speed in a vacuum, about 3.0 × 10⁸ m/s; their speed in air is approximately the same.
• Our eyes detect only visible light, a small part of the spectrum.

A heater transfers energy to your skin by infrared radiation. Sunlight transfers energy to a solar panel. These examples do not require air particles to carry the wave.

8. Draw a ray as it crosses a boundary

1. Draw the boundary and mark where the ray meets it.
2. Draw a dashed normal at right angles to the boundary.
3. Add the incident ray and a direction arrow.
4. For air into glass at an angle, draw the refracted ray closer to the normal. For glass into air, draw it farther from the normal.
5. Measure angles from the normal. If the ray enters along the normal, its direction does not change.

Drawing these ray diagrams is shared Foundation and Higher content. Practise using a ruler and protractor on paper.

9. Higher only: explain refraction using speed

1. One side of an angled wavefront enters glass first.
2. That side slows before the other side, so the wavefront turns.
3. The ray bends towards the normal.
4. Frequency is unchanged. Since v = fλ, a lower speed means a shorter wavelength.

Different substances may absorb, transmit, reflect or refract different wavelengths by different amounts. A material that transmits visible light need not transmit infrared in the same way.

10. Required practical 21: surface finish matters

Warm objects emit infrared radiation. Surfaces can absorb infrared too. Compare finishes fairly rather than comparing two completely different objects.

1. Use identical containers with different surface finishes, equal water volumes and the same starting temperature.
2. Place them equal distances from an infrared lamp. Heat for the same time.
3. Measure temperature rise. Repeat and calculate a mean; swap positions to check for uneven lamp heating.

1. Fill a surface-coated cube with hot water so its faces have approximately the same temperature.
2. Keep detector distance and angle the same for each face.
3. Record readings, allow them to settle, and repeat. Monitor temperature as the cube cools.

Safety: teacher supervision, eye protection, stable containers, care with hot water and lamps. Do not touch hot surfaces.

11. Where do electromagnetic waves come from?

Changes involving electrons in atoms can generate or absorb electromagnetic radiation. Changes in atomic nuclei can also release radiation: gamma rays come from changes in the nucleus.

Higher only: radio circuits

Higher only: oscillations in electrical circuits can produce radio waves. When a receiving circuit absorbs those waves, an alternating current may be induced at the same frequency as the absorbed wave.

12. Use radiation carefully: type and dose both matter

Radiation dose is a measure of the risk of harm from exposure. When dose is given in sieverts (Sv), remember 1 Sv = 1000 mSv. You do not need to recall the unit from memory, but you must use supplied units correctly.

Use evidence, not “all radiation is dangerous” or “a small dose is completely safe”. A medical team weighs the benefit of an image or treatment against the radiation risk. Reduce unnecessary exposure.

13. Choose the wave for the job

The applications are shared content. Explaining why each wave is suitable is Higher-only. Total internal reflection calculations and lens diagrams are not needed for this Trilogy topic.

Choose measurements and justify your method

A calibrated ripple image shows five crest-to-crest gaps across 12 cm. Thirty complete waves pass a point in 10 s. A second group measures only one gap with the same ruler.

Draw and check on paper

1. Draw a transverse wave and label amplitude from the centre line and wavelength between adjacent crests.
2. Draw an air–glass boundary and a normal. Add a ray entering at an angle, with arrows and a correctly bent refracted ray.
3. Higher only: add wavefronts perpendicular to the rays. Show their reduced spacing in glass and explain the change in speed.

Use a ruler. Your website answers are typed; these drawing exercises are for your notebook or a tutoring whiteboard.

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One final thought

Use a scientific idea and a specific example in your reflection.