1.Define power of a lens.Give its unit?

The power of a lens is a measure of the degree of convergence (or) divergence the lens produces on the light falling on it.

The power of a lens P is the reciprocal of its focal length in meter.

The unit of power is dioptre (D)

1 D = 1 m^–1.

2.Why does sky appear blue in colour?

During day time, violet colour which has the shortest wavelength gets more scattered then the other colours. The next scattered colour is blue. As our eyes are more sensitive to blue colour than violet colour, the sky appears blue during day time.

3.What is Rayleigh’s scattering?

If the scattering of light is by atoms and molecules which have size a very much less than that of the wave length λ of light, (a<<λ),then the scattering is called Rayleigh’s scattering

4.State Rayleigh’s scattering law?

The intensity of light scattered in Rayleigh’s scattering is inversely proportional to fourth power of wavelength.

5.What is the reason for reddish appearance of sky during sunset and sunrise?

During sunrise and sunset, the light from sun travels a greater distance through the atmosphere. Hence, the blue light which has shorter wavelength is scattered away and the red light which has longer wavelength and less-scattered manages to reach our eye. This is the reason for the reddish appearance of sky during sunrise and sunset

6.What are the conditions to achieve total internal reflection?

• light must travel from denser to rarer medium,
• angle of incidence in the denser medium must be greater than critical angle (i>i_c ).

7.Reason for glittering of Diamond?

Diamond appears dazzling because of the total internal reflection of light that happens inside the diamond. The refractive index of diamond is about 2.417. It is much greater than the refractive index of ordinary glass which is about only 1.5.

2.Pure water has a refractive index is 1.33.what is the speed of light through it?Ln6 Example Pg13