The radii of curvature of a double convex lens are 4 cm and 8 cm . If the refractive index of the material of the lens is 1.5 , the focal length of the lens is nearly.

A ray is incident from a medium of refractive index 2 into a medium of refractive index 1. The critical angle is

Assertion (A) The focal length of lens does not change when red light is replaced by blue light.

Reason (R) The focal length of lens does not depend on colour of light used.

Light of wavelength $$300 \mathrm{~nm}$$ in medium $$A$$ enters into medium $$B$$ through a plane surface. If the frequency of light is $$5 \times 10^{14} \mathrm{~Hz}$$ and the ratio of speed in medium $$A$$ to that in medium $$B$$ is $$4 / 5$$, the absolute refractive index of medium $$B$$ is