Question

Sketch the emergent wavefront.

Answer 1

Hint: Wavefront is defined as the locus of all the particles of a medium vibrating in the same phase at a given instant. The shape of a given wavefront depends upon the formation of disturbance. Huygens’s principle gave a brief description of the wavefront and used to find the new position of the given wavefront at any instant if its present status is known.

Complete step by step answer:
Here we consider the example of prism to sketch the emergent wavefront. Consider a plane wavefront PQ incident on a glass prism ABC having a small angle of refraction. According to Huygens’s principle, each point on a wavefront PQ acts as a new disturbance center. These centers of disturbances send out secondary wavelets. The different secondary wavelets will travel different distances through a prism.

From point P, suppose the secondary wavelet will travel almost the whole distance in air before its incident on a prism A. After traveling a very small distance in the prism, this wavelet emerges and travels in the air. The time during which the wavelets travel from B to C in glass prism is equal to BC/v. Where v is the speed of light in the glass, at the same time, the wavelet from P will travel almost the whole distance in the air and reaches point C’ such that AC’ > BC. Thus points C’ and C are in the same phase. Join CC’, we get a plane wavefront CC,’ which is known as the emergent plane wavefront.

Note:
According to Huygens’s principle:
i) Each source of light is a center of disturbance from which waves spread in all directions. All particles are equidistant from the source and vibrating in the same phase lies on a wavefront surface.
ii) Every point on a wavefront is a source of new disturbance that produces secondary wavelets. These wavelets travel with the speed of light in all directions in that medium.