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Practice questions · Diffraction in detailEXTENDED

Draw the wavefronts. Keep the spacing.

Eight original Cambridge-style questions. Three ask you to draw wavefronts through a gap and around an edge, the drawings students most often get wrong. Sketch on paper first, then reveal the model diagram to compare.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Three rules for drawing diffraction

Before you sketch any diffracted wave, fix these in your mind.

  1. Spacing never changes. The wavefronts after the gap or edge must be exactly as far apart as before it. Diffraction does not change the wavelength.
  2. Wavefronts curve in the direction of travel. They bulge forward, away from the gap or edge, like ripples spreading outward, never back toward the source.
  3. At an edge, the wave wraps into the shadow. The edge acts like a new source. The wave bends around the corner into the region behind the obstacle, it does not leave a perfectly sharp shadow.
01
[3 marks]

A wave passes through a gap in a barrier.

(a) State the condition for the wave to diffract as strongly as possible. [1] (b) Describe what the diffraction pattern looks like when the gap is much wider than the wavelength. [2]

(a) The gap width is approximately equal to the wavelength of the wave. ✓

(b) The waves pass through almost straight, staying as plane (straight) wavefronts, with only slight curving at the two edges of the gap. ✓✓

Strong diffraction needs gap ≈ wavelength. A wide gap gives weak diffraction.

02
Draw
[3 marks]

The diagram shows plane water waves in a ripple tank approaching a gap. The gap width is approximately equal to the wavelength of the waves.

On a copy of the diagram, draw the wavefronts after they have passed through the gap.

draw here
Plane waves approaching a gap equal in width to the wavelength.

Your drawing should show semicircular wavefronts spreading out from the gap, like ripples from a point source.

Equal spacing. Curving forward. Centred on the gap.

Check with a ruler: the gaps between your curves must match the spacing of the incoming waves. If they grow wider, that is the most common and most penalised mistake.

03
Draw
[4 marks]

The diagram shows plane water waves approaching the straight edge of a long obstacle. To the right of the edge the water is open; behind the obstacle is the shadow region.

On a copy of the diagram, draw the wavefronts after they have passed the edge. Show clearly what happens both in the open region and in the shadow region behind the obstacle.

OBSTACLE edge shadow region draw here
Plane waves approaching the edge of an obstacle.

In the open region the wavefronts carry on straight. As they pass the edge they bend around the corner into the shadow region behind the obstacle.

OBSTACLE shadow region
Straight in the open. Curving around the edge into the shadow.
  • Open region: wavefronts stay straight and keep moving forward.
  • Near the edge: they curl around the corner into the shadow.
  • Spacing stays the same everywhere.

Two classic mistakes: drawing a perfectly sharp shadow with nothing behind the obstacle, and letting the wavelength change. The shadow is never perfectly sharp, because the wave wraps around the edge.

04
Analysis
[3 marks]

As a water wave diffracts through a gap, state what happens to each of the following, and give a reason.

(a) its wavelength [1] (b) its speed [1] (c) its frequency [1]

(a) Wavelength stays the same. ✓

(b) Speed stays the same. ✓

(c) Frequency stays the same. ✓

The wave stays in the same medium (same water), so none of its properties change. Diffraction changes only the shape and direction of the wavefronts, not the wave itself.

Contrast with refraction, where the medium DOES change, so speed and wavelength change. Diffraction keeps the wave in one medium, so nothing about the wave changes.

05
Analysis
[4 marks]

A person stands behind a tall, wide wall. They cannot see a car on the far side, but they can clearly hear its engine.

Explain this observation in terms of diffraction and wavelength.

  • Both sound and light diffract (bend) around the edges of the wall.
  • The amount of diffraction depends on how the wavelength compares with the size of the obstacle.
  • Sound has a long wavelength, comparable to the size of the wall, so it diffracts strongly and bends around into the shadow where the person stands. They hear it.
  • Light has an extremely short wavelength, far smaller than the wall, so it barely diffracts. It travels almost straight and leaves a sharp shadow, so the person cannot see the car.

Same structure as the corner question: both diffract, the rule is wavelength vs obstacle size, then apply to each. The marks are in the comparison.

06
Analysis
[3 marks]

A house sits in a valley, with a large hill between it and a distant transmitter. The residents can receive long-wave radio (wavelength about 1500 m) clearly, but cannot receive television signals (wavelength about 0.5 m).

Explain, in terms of diffraction, why the long-wave radio reaches the house but the television signal does not.

  • The long-wave radio has a wavelength (about 1500 m) comparable to, or larger than, the size of the hill. So it diffracts strongly and bends around the hill into the valley.
  • The television signal has a much shorter wavelength (about 0.5 m), far smaller than the hill. So it diffracts very little and travels almost straight, leaving the valley in the hill's shadow.

The longer the wavelength compared with the obstacle, the more the wave bends around it. This is why long-wave radio has such good coverage over hilly ground.

07
Draw
[4 marks]

Waves of the same wavelength are sent through two different gaps in separate experiments. Gap A is much wider than the wavelength. Gap B is approximately equal to the wavelength.

For each gap, sketch the pattern of the wavefronts on the far side. Make clear how the two patterns differ.

Gap A (wide)
Gap B (equal)
  • Gap A (wide): waves pass through almost straight, with only slight curving at the edges.
  • Gap B (equal): waves spread out into strong semicircular wavefronts, like a point source.
  • In both, the spacing of the wavefronts is unchanged.

The narrower gap (closer to the wavelength) gives much more spreading. Same wavelength, very different patterns.

08
Analysis
[3 marks]

A student draws the wavefronts spreading out from a narrow gap. In their drawing, the curved wavefronts get further and further apart as they move away from the gap.

State what is wrong with the drawing, and explain what the student has misunderstood.

  • The wavefronts should stay the same distance apart, not get further apart.
  • The distance between wavefronts is the wavelength, and diffraction does not change the wavelength.
  • By drawing them spreading apart, the student has wrongly shown the wavelength (and therefore the speed) increasing, which does not happen.

Diffraction changes the shape and direction of the wavefronts only. The wavelength is fixed. Always check your spacing with a ruler.