Diffraction

AS Level Physics · Superposition. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Diffraction is the spreading of a wave when it passes through a gap or round the edge of an obstacle. All waves do it. In a ripple tank you can watch straight wavefronts pass through a gap in a barrier and come out curved.

How much the wave spreads depends on the size of the gap compared with the wavelength. When the gap is about the same size as the wavelength, the waves spread out almost as semicircles. When the gap is many wavelengths wide, the waves go nearly straight on with only slight bending at the edges.

Diffraction changes the direction of travel only. The wavelength, frequency and speed stay the same.

Rules to remember

Diffraction: the spreading of a wave as it passes through a gap or round an edge
Most diffraction when gap width ≈ wavelength
Gap much wider than the wavelength: very little diffraction
Wavelength, frequency and speed are unchanged by diffraction
At the same speed, higher frequency means shorter wavelength (λ = v/f), so less diffraction through the same gap
Long wavelengths bend round large obstacles: sound (about 1 m) bends round a corner, light (about 5 × 10⁻⁷ m) does not

Common mistake

Students say that diffraction changes the wavelength. It does not: draw the wavefronts after the gap with the same spacing as before it.

Worked example

Water waves of frequency 8.0 Hz travel at 0.24 m s⁻¹ in a ripple tank. They can pass through a gap of width 3.0 cm, 12 cm or 30 cm. Which gap gives the most diffraction?

Wavelength λ = v ÷ f = 0.24 ÷ 8.0 = 0.030 m = 3.0 cm
Diffraction is greatest when the gap width is closest to the wavelength
The 3.0 cm gap equals the wavelength

Answer: The 3.0 cm gap

Practice questions

Try each one, then open the answer.

1. What is meant by diffraction?

A
the spreading of a wave as it passes through a gap or around the edge of an obstacle
B
the change in direction of a wave as it enters a different medium
C
the superposition of two coherent waves
D
the reflection of a wave from a surface

Show answer

Answer: A. Diffraction is the spreading of waves at a gap or an edge. The change of direction on entering a new medium is refraction.

2. Waves diffract as they pass through a narrow gap. Which property of the waves is the same after the gap as before it?

A
the wavelength
B
the direction of travel
C
the amplitude at every point
D
the shape of the wavefronts

Show answer

Answer: A. Diffraction does not change the medium, so speed, frequency and wavelength stay the same. The wavefronts become curved and spread out, so direction and amplitude change.

3. Microwaves of wavelength 3.0 cm are directed at a gap in a metal sheet. For which gap width would there be very little diffraction?

A
1.5 cm
B
3.0 cm
C
6.0 cm
D
1.0 m

Show answer

Answer: D. When the gap is much wider than the wavelength (1.0 m is over 30 wavelengths) the waves pass through almost without spreading.

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Diffraction

The idea

Rules to remember

Common mistake

Worked example

Water waves of frequency 8.0 Hz travel at 0.24 m s−1 in a ripple tank. They can pass through a gap of width 3.0 cm, 12 cm or 30 cm. Which gap gives the most diffraction?

Practice questions

1. What is meant by diffraction?

2. Waves diffract as they pass through a narrow gap. Which property of the waves is the same after the gap as before it?

3. Microwaves of wavelength 3.0 cm are directed at a gap in a metal sheet. For which gap width would there be very little diffraction?

Read the full notes

Keep going

Water waves of frequency 8.0 Hz travel at 0.24 m s⁻¹ in a ripple tank. They can pass through a gap of width 3.0 cm, 12 cm or 30 cm. Which gap gives the most diffraction?