FIG. 16.10 The light coming out of a bright room into a dark hallway spreads as it diffracts
b
FIG. 16.11 The amount that a wave diffracts depends on the relative size of gap to wavelength
Interference
Since waves are disturbances in space, they can interfere with each other when they meet. They may add to one another (constructive interference) or cancel out parts of each other (destructive interference).
NOTE
In 1803 Thomas Young proved that light is a wave by showing that it undergoes interference. Two light waves interfere to produce light and dark fringes on a screen (see Chapter 18).
If the result of two waves causes a new wave of differing amplitude, interference has taken place. This can have an interesting effect in light and sound waves and other types of waves.
In interference waves combine to form a resultant amplitude made up of each wave’s individual amplitude.
Constructive interference is when waves combine to form an amplitude greater than each individual wave’s amplitude.
Destructive interference is when waves combine to form an amplitude smaller than each individual wave’s amplitude.
FIG. 16.12 A silencer on a car’s exhaust can use destructive interference to reduce the noise produced by the car’s engine
If you play two identical sounds and get them to meet, you can create anything from double the amplitude (louder) to zero amplitude (silence). This phenomenon has been used in many areas such as environmental noise control on car exhausts (Fig. 16.12) and even in riot-control weapons.
Two waves of the same frequency that are in step or a fixed amount out of step
are known as coherent waves. If coherent waves meet, they will produce an interference pattern (Figs. 16.13 and 16.14).
