the spreading of a wave through a gap or round an edge. The others are refraction, reflection and superposition.

When is diffraction through a gap greatest?

when the gap is about the same size as the wavelength. A wide gap gives little spreading.

What happens to the wavelength of a wave when it is diffracted?

It stays the same. Only the shape of the wavefronts changes.

Water waves of wavelength 2 cm pass through gaps of different widths. Which gap gives the most spreading?

2 cm. The gap that matches the wavelength.

Which waves bend furthest round a hill?

long-wavelength radio waves. The longer the wavelength, the more the wave diffracts round a large obstacle.

Diffraction · Cambridge A Level Physics 9702 revision

In plain words

Waves don't just go straight through a gap: they spread out on the far side. That is why you can hear someone round a corner but can't see them. Sound waves are about the size of a doorway, so they spread right round it. Light waves are tiny, so they hardly spread at all.

4 things to know

Diffraction is the spreading of a wave when it passes through a gap or round the edge of an obstacle.
The spreading is greatest when the gap is about the same size as the wavelength. A gap much wider than the wavelength gives very little spreading.
Diffraction does not change the wavelength, the frequency or the speed of the wave.
One way to picture it (Huygens' construction): every point on a wavefront acts as a source of new circular wavelets, and the next wavefront is the line that joins them up. At a narrow gap only a few of those sources get through, so the wavefront comes out curved.

Tips and tricks

When you draw diffraction, keep the wavelength the same on both sides of the gap.
A narrower gap gives more spreading; a wider gap gives less.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Diffraction: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What is diffraction?
- the spreading of a wave through a gap or round an edge (the answer)
- the bending of a wave when its speed changes
- the bouncing of a wave off a surface
- the adding together of two waves
The others are refraction, reflection and superposition.
When is diffraction through a gap greatest?
- when the gap is much wider than the wavelength
- when the gap is about the same size as the wavelength (the answer)
- when the gap is closed
- it is always the same
A wide gap gives little spreading.
What happens to the wavelength of a wave when it is diffracted?
- It stays the same. (the answer)
- It increases.
- It decreases.
- It halves.
Only the shape of the wavefronts changes.
Water waves of wavelength 2 cm pass through gaps of different widths. Which gap gives the most spreading?
- 2 cm (the answer)
- 10 cm
- 50 cm
- 1 m
The gap that matches the wavelength.
Which waves bend furthest round a hill?
- long-wavelength radio waves (the answer)
- microwaves
- visible light
- X-rays
The longer the wavelength, the more the wave diffracts round a large obstacle.

5 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

3 questions, 5 marks. Write your answers on paper, then check them.

Diffraction

Cambridge International AS & A Level Physics 9702 · 5 marks · papermunch.org

Name ______________________________ Date ______________

Explain why sound diffracts through a doorway but light does not noticeably do so.[2]
Show answerHide answer
The wavelength of sound is about the same size as the doorway, so it spreads a great deal. The wavelength of light is far smaller than the doorway, so it spreads very little.
I got this one
Describe how diffraction can be shown with water waves.[2]
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In a ripple tank, send straight waves towards a gap between two barriers. The waves spread out beyond the gap. Making the gap narrower, closer to the wavelength, makes them spread more.
I got this one
A gap stays the same width and the wavelength of the waves is increased. State what happens to the amount of diffraction.[1]
Show answerHide answer
It increases.
I got this one

Answers: Diffraction

1. The wavelength of sound is about the same size as the doorway, so it spreads a great deal. The wavelength of light is far smaller than the doorway, so it spreads very little.
2. In a ripple tank, send straight waves towards a gap between two barriers. The waves spread out beyond the gap. Making the gap narrower, closer to the wavelength, makes them spread more.
3. It increases.