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