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3.2.1Waves

Light and Sound Waves — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 9 of 14

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision
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Part 1 of 7Two very different waves

Light and sound are both waves, but they are very different in nature. Light is an electromagnetic wave: a transverse oscillation of electric and magnetic fields that needs no medium. Sound is a mechanical longitudinal wave: vibrations of particles in a material passed on as compressions and rarefactions. A medium must be present for sound to travel.

Part 2 of 7Speeds

The speed of light in a vacuum is 3.0 × 108 m s−1, and it is slightly less in air and much less in glass or water.

Sound is much slower.
In air at room temperature it is about 340 m s−1, in water about 1500 m s−1 and in steel about 5000 m s−1.
Faster
Unlike light, sound travels faster in denser solids than in gases, because the particles are closer together and pass on vibrations more quickly.
Light travels about a million times faster than sound in air.
This is why you see lightning before you hear thunder.

The speed of sound in air increases slightly as the temperature rises, as the particles move faster and pass on the disturbance more readily.

Part 3 of 7What both waves can do

Reflected
Both can be reflected: light by mirrors, sound by hard smooth surfaces as echoes.
Refracted
Both can be refracted when their speed changes in a new medium.
Diffracted
Both can be diffracted when passing through gaps or round edges.
  • Sound has wavelengths of the order of metres, similar to doorways, so it diffracts easily and can be heard round corners.
  • Light has wavelengths below a micrometre, so diffraction is not noticeable with everyday objects, and light seems to travel in straight lines.

Part 4 of 7Differences summarised

PropertyLightSound
TypeTransverse, electromagneticLongitudinal, mechanical
Medium neededNoYes
Speed in airAbout 3 × 108 m s−1About 340 m s−1
Speed in denser materialSlowerFaster
Typical wavelength400–700 nmAbout 0.02 m to 17 m (audible)

Part 5 of 7Using the speed difference

Because sound is so much slower than light, the time lag between a flash and a bang can be used to find a distance.

Using the speed difference
In a storm, a 3 s gap between lightning and thunder means the lightning is about 340 × 3 ≈ 1000 m away; the light arrives in a negligible time.
Echoes are used in a similar way by ships using sonar: the time between a pulse and its echo is measured and distance is found from distance = speed × time ÷ 2.
  • For the behaviour of the two waves at boundaries see Reflection and Refraction, and for sound measurements see Investigating the Speed of Sound.

Part 6 of 7Reflection, refraction and sound in practice

Both waves change direction at boundaries, but the effects are noticed in different ways.

1
Sound reflecting from a large hard wall gives an echo,
  • and the delay depends on the distance;
    • in a small room, the echo arrives so quickly that it merges with the original sound,
  • which is heard as reverberation.
2
Soft furnishings and curtains absorb sound, reducing echoes.
3
Light is reflected strongly from smooth shiny surfaces and diffusely from rough ones.
4
Sound refracts too.
5
On a cold evening, the air near the ground can be cooler than the air above it,
  • so sound travels more slowly near the ground and is bent downwards,
  • which is why distant sounds seem clearer at night.
6
Light refracts when it enters water or glass and this changes the apparent depth of objects.

Both waves transfer energy, which is why loud sounds can shake windows and why sunlight warms surfaces.

Reflection, refraction and sound in practice
The amplitude sets the energy: a larger amplitude light wave is brighter and a larger amplitude sound wave is louder.
Intensity decreases with distance from a point source as the energy spreads over a larger area,
  • which is why a distant lamp seems dimmer and a distant voice quieter.

Part 7 of 7Worked examples

  1. A lightning flash is seen and the thunder is heard 6.0 s later. How far away is the storm? (speed of sound 340 m s−1)

  2. A sound of frequency 850 Hz travels in air at 340 m s−1. Find its wavelength.

  3. A boat sends a sound pulse down and receives the echo 0.080 s later. The speed of sound in water is 1500 m s−1. Find the depth.

Key terms

TermMeaning
Longitudinal waveParticle oscillations parallel to wave direction
Electromagnetic waveOscillating electric and magnetic fields; needs no medium
EchoReflection of sound
SonarUsing echoes of sound to find distances underwater
DiffractionSpreading of waves round obstacles and through gaps

Avoid these mistakes

1
Saying sound travels faster in air than in water – it is faster in denser media.
2
Thinking sound can travel in a vacuum.
3
Saying light needs particles to travel.
4
Forgetting that the echo distance must be halved.
5
Believing light travels instantly – it is just very fast.

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