Interesting and unexpected results are obtained when straight waves are incident on an opening formed between two vertical metal barriers placed in the ripple tank. If the opening is a wide one compared with the wavelength of the waves, they will pass through in parallel straight lines, though we cannot fail to notice a slight bending round at the edges. Matters are entirely different when the opening is a narrow one about the same order of width as the wavelength. The wavefront now emerges with a pronounced circular shape and the waves spread out in all directions from the opening. This effect is called diffraction (see Fig. 26.9).
Light behaves in a similar manner. If a parallel beam of light falls on a screen after passing through a wide slit the diffraction is negligible; very much less, in fact, than in the case of water waves. We get a sharp-edged patch of light on the screen so that it appears that the light is travelling in straight lines. Indeed, we have already mentioned the straight line propagation of light in chapter 21, and we saw later that it worked very well in explaining the formation of images by mirrors and lenses. On the other hand, if light passes through a very narrow slit it can be shown to spread out in a manner similar to the water waves in Fig. 26.9. The reason why we never notice the diffraction of light in everyday life is that the wavelength of light is exceedingly small and so the effect is unobservable except when the light passes through very narrow openings. Even in the laboratory we have to go to some trouble to demonstrate it. However, in due course we shall show how diffraction can be used to measure the wavelength of light.