Refraction of waves

When waves travel from one medium to another, they tend to change their travelling speed. This phenomenon is known as refraction of waves. Refraction of waves occurs because the speed of a wave depends on the medium through which the wave is travelling. If the medium is changed, the speed of the wave will also change. The change in speed results in a change in the wavelength of the wave.

When water waves travel from a deep part to a shallow part, the wavelength decreases as illustrated in Figure 1.18. However, the frequency of the waves does not change. Since velocity, v, is given by λf, the velocity of the waves decreases with decrease in wavelength.

Figure 1.18: Change in the wavelength of water waves

The ratio —— is equal to the ratio of the velocities of water waves in the deep water, v to that in the shallow water, v₁

The velocity of water waves is higher in the deep water than in the shallow water.

Refractive index

The refraction of water waves can be further observed if the boundary between the deep and shallow regions is at a cartain angle to the incident wavefronts. In this case the deep region acts as the first medium while the shallow region acts as the second medium. If in Activity 1.2, the glass plate was placed at some angle, water waves would be refracted as shown in Figure 1.19.

Figure 1.19: Refraction of water waves

Experimental observations have shown that water waves obey Snell’s Law of refraction. That is,

Where n is the refractive index of the second medium relative to the first medium, i is the angle of incidence and r is the angle of refraction.

The refractive index, n is also the ratio of the velocity of the wave in the first medium to that in the second medium, that is;

Applications of refraction of waves

As in the case of wave reflection, wave refraction is also useful in many activities. The following are some applications of refraction of waves:

1. Refraction is used in optical instruments which focus or spread light. These include cameras, microscopes and telescopes.
2. Spectacles worn by people with visual impairment use the principle of refraction of light.
3. Since every material has its own value of refractive index, purity of a material can be identified by determining the refractive index of the material.