Question

Define the term wave front. State Huygens’s principle.
Consider a plane wave front incident on a thin convex lens. Draw a proper diagram to show how the incident wave front traverses through the lens and after refraction focuses on the focal point of the lens, giving the shape of the emergent wave front.

Answer 1

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When source of light is a point source, the wave front is a spherical wavefront.

Huygens Principle:

(1) Every point on the given wave front (called primary wave front) acts as a fresh source of new disturbance, called secondary wavelets, which travel in all directions, with the velocity of light, in the medium.

(2) A surface touching these secondary wavelets, tangentially in the forward direction at any instant gives the new wave front at that instant. This is called secondary wavefront.

OR

(i) The frequency and time period of an electromagnetic wave depends only on the source which produces it. The frequency is independent of the medium through which it travels. But the speed and wavelength depends on the medium through which the wave travels. Because of this, the frequency and time period of sound wave do not change due to change in medium.

(ii) Speed decreases due to decrease of wavelength of wave but energy carried by the light wave depends on the amplitude of electric field vector.

(iii) The number of photons emitted from metal surface depends only on the intensity of the incident light and is independent of its frequency. The number of photons emitted per second is directly proportional to the intensity of the incident light.

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