Single slit diffraction

We have seen lots of evidence that light behaves as a wave. Other experiments show just as convincingly that light behaves as a particle. It is interesting to find out what happens when we try to combine the two aspects. The challenge is to devise experiments which show light as a beam of particles and then to determine if these particles also show wavelike behaviour.

Figure 14.3 Intensity distribution of 'photon hits' on the photographic film. Photons make the same pattern as waves when they come out of a thin slit! (cf. Figure 8.6.)

Working in a completely dark room, we reduce the intensity of a beam of light until the photons arrive 'one at a time'. In our first simple experiment we shine this beam on a narrow slit with a photographic plate behind it, in place of a screen. With such a low light intensity we will have to wait for quite a long time. Eventually, when we develop the film, we find exactly the same pattern as we got when using a beam of normal intensity.

This result is somewhat surprising, but we can probably construct some explanation. Perhaps, as the photons come through one by one, they are attracted by the edges of the slit, and not all continue in a straight line. That might explain the width of the centre of the diffraction pattern. The secondary maxima are more difficult to explain — but if we use a lot of imagination we might come up with something!

0 0

Post a comment