A ray of light passes from the vacuum into a medium of refractive index 'n'. If the angle of incidence is twice the angle of refraction, then the angle of incidence in terms of refractive index is

A convex lens having refractive index 1.6 has focal length 12 cm, when it is in air. The focal length of that lens when placed in water is (refractive index of water = 1.28)

Three identical polaroids $P_1$, $P_2$ and $P_3$ are placed one after another. The pass axis of $P_2$ and $P_3$ are inclined at angle of $60^\circ$ and $90^\circ$ with respect to axis of $P_1$. The source has an intensity $I_0$. The intensity of light finally coming out is

In Young's double slit experiment, two slits are illuminated with a light of wavelength $\lambda$. The line joining $A_1P$ is perpendicular to $A_1A_2$ as shown in figure. If the first minimum is detected at P, the value of slits separation 'a' will be
(D = distance between source and screen)