If two slits in Young's experiment are $$0.4 \mathrm{~mm}$$ apart and fringe width on a screen $$200 \mathrm{~cm}$$ away is $$2 \mathrm{~mm}$$ the wavelength of light illuminating the slits is

The distance of moon form the earth is $$3.8 \times 10^5 \mathrm{~km}$$. Supposing that the eye is most sensitive to the light of wavelength $$550 \mathrm{~nm}$$, the separation of two points on the moon that can be resolved by a $$500 \mathrm{~cm}$$ telescope is

Unpolarised light falls on two polarising sheets placed one on top of other. If the intensity of transmitted light is one fourth of the incident light, the angle between them is