Diffraction

When a wave hits an obstacle it does not simply go straight past, it bends round the obstacle. The same type of effect occurs at a hole - the waves spread out the other side of the hole. This phenomenon is known as diffraction and examples of the diffraction of plane waves are shown in Figure 1.



The effects of diffraction are much more noticeable if the size of the obstacle is small (a few wavelengths across), while a given size of obstacle will diffract a wave of long wavelength more than a shorter one.



Diffraction can be easily demonstrated with sound waves or microwaves. It is quite easy to hear a sound even if there is an obstacle in the direct line between the source and your ears. By using the 2.8 cm microwave apparatus owned by many schools very good diffraction effects may be observed with obstacles a few centimetres across.


One of the most powerful pieces of evidence for light being some form of wave motion is that it also shows diffraction. The problem with light, and that which led Newton to reject the wave theory is that the wavelength is very small and therefore diffraction effects are hard to observe. You can observe the diffraction of light, however, if you know just where to look.



The coloured rings round a street light in frosty weather, the coloured bands viewed by reflection from a record and the spreading of light round your eyelashes are all diffraction effects. Looking through the material of a stretched pair of tights at a small torch bulb will also show very good diffraction. A laser will also show good diffraction effects over large distances because of the coherence of laser light.



Diffraction is essentially the effect of removing some of the information from a wave front; the new wave front will be altered by the obstacle or aperture. Huygens' theory explained this satisfactorily.



Grimaldi first recorded the diffraction of light in 1665 but the real credit for its scientific study must go to Fresnel, Poisson and Arago, working in the late eighteenth and early nineteenth centuries.

Fresnel and Fraunhofer diffraction

We can define two distinct types of diffraction:
(a) Fresnel diffraction is produced when light from a point source meets an obstacle, the waves are spherical and the pattern observed is a fringed image of the object.
(b) Fraunhofer diffraction occurs with plane wave-fronts with the object effectively at infinity. The pattern is in a particular direction and is a fringed image of the source.

Fresnel diffraction

Fresnel diffraction can be observed with the minimum of apparatus but the mathematics are complex. We will therefore only treat it experimentally here.

If a razor blade is placed between the observer and a point source of monochromatic light, dark and bright diffraction fringes can be seen in the edges of the shadow. The same effects can be produced with a pinhead, when a spot of light will be seen in the centre of the shadow.

Fresnel was unhappy about Newton's explanation of diffraction in terms of the attraction of the light particles by the particles of the solid, because diffraction was found to be independent of the density of the obstacle: a spider's web, for example, gave the same diffraction pattern as a platinum wire of the same thickness. The prediction and subsequent discovery of a bright spot within the centre of the shadow of a small steel ball was final proof that light was indeed a wave motion.