Diffraction at A Level

Diffraction is the spreading of waves when they pass through a gap or by an edge. Particles would not do that so diffraction is evidence of the wave nature of light.

This general property of all waves is very important in the design of optical instruments, such as cameras, microscopes and telescopes.

For example, when a telescope is used to observe planets, we can often see features that are not evident when observed directly. This is partly because less diffraction occurs when waves pass through a wide gap than through a narrow gap. Therefore, because a telescope is much wider than the eye pupil, much less diffraction occurs when using a telescope than when observing with the unaided eye.


Diffraction of water waves through a gap can be observed using a ripple tank.

This arrangement shows that the diffracted waves become more circular if:

the gap is made narrower, or

the wavelength is made larger.

Close examination of the diffracted waves reveals that the each diffracted wavefront has breaks either side of the centre. These breaks are due to waves diffracted by adjacent sections on the gap being out of phase and cancelling each other out in certain directions.

Diffraction of light by a single slit can be demonstrated by directing a parallel beam of light at the slit (a laser is good to use).

The diffracted light forms a pattern that can be observed on a white screen.

You see 'fringes' - bands of light (which from a laser are red) and no light (which on the diagram are shown in black.

The pattern shows a central bright fringe with further dark and bright fringes either side. The intensity of the fringes is greatest at the centre of the central fringe. The central fringe is twice as wide as the others.


the central fringe is twice as wide as each of the outer fringes (measured from minimum to minimum intensity),

the peak intensity of each fringe decreases with distance from the centre,

each of the outer fringes is the same width,

the outer fringes are much less intense than the central fringe.

 If the single slit pattern is observed:

using different sources of monochromatic light in turn, the observations show that the greater the wavelength, the wider the fringes,

using an adjustable slit, the observations show that making the slit narrower makes the fringes wider.

It can be shown theoretically that using light of wavelength , the width Wc of the central fringe observed on a screen at distance D from the slit is given by:

Wc = 2D( /a)

Now look at the page on Diffraction Gratings



Now try some multiple choice questions on diffraction.