The refractive index of the material of a small angled prism is 1.6. If the angle of minimum deviation is $4.2^{\circ}$, the angle of the prism is

The Brewster angle for air to glass transition of light is (Refractive index of glass $=15$ )

When an object of height 12 cm is placed at a distance from a convex lens, an image of height 18 cm is formed on a screen. Without changing the positions of the object and the screen. If the lens is moved towards the screen, another clear image is formed on the screen. The height of this image is

A thin plano-convex lens of focal length 73.5 cm has a circular aperture of diameter 8.4 cm . If the refractive index of the material of the lens is $\frac{5}{3}$, then the thickness of the lens is nearly