Find the focal length of a lens of power $–2.0 \mathrm{D}$. What type of lens is this?

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.