Diffraction is the spreading out of a wave as it passes through a gap or around the edge of an obstacle.

Describe the pattern produced when monochromatic light passes through a single narrow slit onto a screen.

There is a broad, very bright central band with much fainter bands on either side, separated by dark regions, and the outer bands become progressively dimmer further from the centre.

What happens to a single slit diffraction pattern when the slit is made narrower?

The whole pattern spreads out, so the central band becomes wider and the light is spread more thinly, making the pattern dimmer overall.

Under what condition is diffraction at a gap most pronounced?

Diffraction is greatest when the width of the gap is comparable with the wavelength of the wave passing through it.

CCEA A-Level Physics 1210

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Superposition, Interference and Diffraction

Diffraction at a single slit

3 things to know
Unit AS 2

Overview

Recall questions on what diffraction is and the pattern a single slit produces, how the central band compares with the others in width and brightness, the effect of narrowing the slit or changing the wavelength, why sound diffracts round a doorway but light does not, the ripple tank demonstration, and the limit diffraction sets on a telescope.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why can sound be heard round the corner of an open doorway while light cannot be seen round it?

Key term

What is diffraction?

Key term

How does the width of the central bright band of a single slit pattern compare with that of the bands beside it?

Questions from this subtopic

Each one with the model answer underneath.

1
What is diffraction?
Model answer
Diffraction is the spreading out of a wave as it passes through a gap or around the edge of an obstacle.
2
Describe the pattern produced when monochromatic light passes through a single narrow slit onto a screen.
Model answer
There is a broad, very bright central band with much fainter bands on either side, separated by dark regions, and the outer bands become progressively dimmer further from the centre.
3
How does the width of the central bright band of a single slit pattern compare with that of the bands beside it?
Model answer
The central band is about twice as wide as each of the others, as well as being far brighter.
4
What happens to a single slit diffraction pattern when the slit is made narrower?
Model answer
The whole pattern spreads out, so the central band becomes wider and the light is spread more thinly, making the pattern dimmer overall.
5
Under what condition is diffraction at a gap most pronounced?
Model answer
Diffraction is greatest when the width of the gap is comparable with the wavelength of the wave passing through it.
6
Why can sound be heard round the corner of an open doorway while light cannot be seen round it?
Model answer
The wavelength of audible sound is comparable with the width of a doorway, so sound diffracts strongly, whereas the wavelength of light is smaller by a factor of about a million and it spreads by an utterly negligible angle.

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