State the principle of reversibility of light.

What will happen to a ray of light when it falls normally on a surface.

What are the characteristics of the image formed when an object is at infinity from a concave lens?

Write down the magnification formula for a lens in terms of object distance and image distance. How does it differ from the corresponding formula for a mirror?

The speed of light in water is$2.25\times {10}^8 \mathrm{m}/\mathrm{s}$. If the speed of light in a vacuum is $3\times {10}^8\mathrm{m}/\mathrm{s}$. Calculate the refractive index of water.

The refractive index of a diamond is $2.42$. What is the meaning of this statement in relation to speed of light?

The refractive index of dense flint glass is $1.65$ and for alcohol it is $1.36$, with respect to air. What is the refractive index of dense flint glass with respect to alcohol?

From the mirror equation $\frac{1}{\mathrm{v}}+\frac{1}{\mathrm{u}}=\frac{2}{\mathrm{R}},$ show that the image in a plane mirror is as far behind the mirror as the object is in front of it.

A convex lens forms a real and inverted image of a needle at a distance of $50 \mathrm{cm}$ from the lens. Where is the needle placed in front of the convex lens so that its image is of the same size as the object? Also find the power of the lens.