Refraction Through A Prism: Prism is a transparent body having three rectangular and two triangular surfaces as shown in fig. 14. 9 (a). The angle of the triangular surface opposite to its base is known as “Angle of prism”. Fig. 14.9 (b) shows the path of a ray of light refracted through a prism. The incident ray PQ strikes the face AB of the prism.

Then on entering the prism the ray bends towards the normal at the point of incidence Q i.e., it bends towards the base of the prism. The refracted ray QR on coming out of the prism, bends away from the normal RN at the point of emergence R i.e., the emergent ray RS bends towards the base of the prism.

Fig. 14.9 (a)-Right-angle-prism

Angle of prism

The incident ray PQ makes an angel of incidence Î and ř is its corresponding angle of refraction in the prism.

According to the law of refraction

incident ray

Where n is the refractive index of the prism. The original direction of incident ray is PQT but it is turned through angle TDS on passing through the prism. The angle TDS is known as angle of deviation. The value of angle of deviation depends upon the value of the angle of incidence. When the angle of incidence is continuously increased from a small value, the angle of deviation first decreases reaches a minimum value and then starts increasing.

The minimum value of the angle of deviation is known as the angle of minimum deviation and is denoted by Dm. The refractive index n of the material of the prism with respect to air can be determined by the following relation:

angle of prism

Where a is the angle of prism and Dm is the angle of minimum deviation.

minimum deviationThe refraction of waves depends on their wavelength. Since the sunlight consists of different colours, the waves of different wavelengths, thus when it passes through a prism then the waves of different wavelengths deviate on different paths, due to this white light disperses in different colours, which is called disperses, and the band of colors which is seen on the screen is called solar spectrum.

Do You Know?

Periscope is a long tube at the two ends of which, two totally reflecting prisms are adjusted. Line rays of light from an object are totally reflected through an angle of 90 degree by each prism. Thus light from a viewed object enters the observer’s eye. Periscope is normally used in tanks and submarines. With the help of periscope. The commander of the submarine sitting in his cabin can see the objects on the surface of the sea. Similarly, a solider sitting inside a tank can see the object outside the tank.