You should be aware that all waves (such as sound and light) can be reflected and refracted. Another wave phenomenon that applies to all waves is that they can be diffracted. Diffraction is the spreading of a wave as it passes through a gap or around an edge. It is easy to observe and investigate diffraction effects using water waves, as shown in Practical Activity 13.1.

Diffraction of sound and light

Diffraction effects are greatest when waves pass through a gap with a width roughly equal to their wavelength of the waves. This is useful in explaining why we can observe diffraction readily for some waves, but not for others. For example, sound waves in the audible range have wavelengths from a few centimetres to a few metres (see Table 12.1). So, we might expect to observe diffraction effects for sound in our environment. Sounds, for example, diffract as they pass through doorways. The width of a doorway is comparable to the wavelength of a sound and so a noise in one room spreads out into the next room.
Visible light has much shorter wavelengths (about $5 \times {10^{ - 7}}m$). It is not diffracted noticeably by doorways because the width of the gap is a million times larger than the wavelength of light. However, we can observe diffraction of light by passing it through a very narrow slit or a very small hole. When laser light is directed onto a slit whose width is comparable to the wavelength of the incident light, it spreads out into the space beyond to form a smear on the screen (Figure 13.5). An adjustable slit allows you to see the effect of gradually narrowing the gap.
You can see the effects of diffraction for yourself by making a narrow slit with your two thumbs and looking through the slit at a distant light source (Figure 13.8). By gently pressing your thumbs together to narrow the gap between them, you can see the effect of narrowing the slit.

PRACTICAL ACTIVITY 13.1

 

Observing diffraction in a ripple tank

A ripple tank can be used to show diffraction. Plane waves are generated using a vibrating bar, and move towards a gap in a barrier (Figure 13.6). Where the ripples strike the barrier, they are reflected back. Where they arrive at the gap, however, they pass through and spread out into the space beyond.
It is this spreading out of waves as they travel through a gap (or past the edge of a barrier) that is called diffraction.
The extent to which ripples are diffracted depends on the width of the gap. This is illustrated in Figure 13.6. The lines in this diagram show the wavefronts. It is as if we are looking down on the ripples from above, and drawing lines to represent the tops of the ripples at some instant in time. The separation between adjacent wavefronts is equal to the wavelength λ of the ripples.
When the waves encounter a gap in a barrier, the amount of diffraction depends on the width of the gap. There is hardly any noticeable diffraction when the gap is very much larger than the wavelength.
As the gap becomes narrower, the diffraction effect becomes more noticeable. It is greatest when the width of the gap is roughly equal to the wavelength of the ripples.

 

Figure 13.7: The extent to which ripples spread out depends on the relationship between their wavelength and the width of the gap. In a, the width of the gap is very much greater than the wavelength and there is hardly any noticeable diffraction. In b, the width of the gap is greater than the wavelength and there is limited diffraction. In c, the gap width is approximately equal to the wavelength and the diffraction effect is greatest.

Diffraction of radio waves and microwaves

Radio waves can have wavelengths of the order of a kilometre. These waves are easily diffracted by gaps in the hills and by the tall buildings around our towns and cities. Microwaves, used by the mobile phone network, have wavelengths of about $10 cm$. These waves are not easily diffracted (because their wavelengths are much smaller than the dimensions of the gaps) and mostly travel through space in straight lines.
Cars need external radio aerials because radio waves have wavelengths longer than the size of the windows, so they cannot diffract into the car. If you try listening to a radio in a train without an external aerial, you will find that FM signals can be picked up weakly (their wavelength is about $3 m$), but AM signals, with longer wavelengths, cannot get in at all.

Question

 

A microwave oven (Figure 13.9) uses microwaves with a wavelength of $12.5 cm$. The front door of the oven is made of glass with a metal grid inside; the gaps in the grid are a few millimetres across.
Explain how this design allows us to see the food inside the oven, while the microwaves are not allowed to escape into the kitchen (where they might harm us).

Explaining diffraction

Diffraction is a wave effect that can be explained by the principle of superposition. We have to think about what happens when a plane ripple reaches a gap in a barrier (Figure 13.10). Each point on the surface of the water in the gap is moving up and down. Each of these moving points can be thought of as a source of new ripples spreading out into the space beyond the barrier. Now we have a lot of new ripples, and we can use the principle of superposition to find their resultant effect. Without trying to calculate the effect of an infinite number of ripples, we can say that in some directions the ripples add together while in other directions they cancel out.