To obtain a virtual and erect image for any position of the object we need:

A convex lens of focal length $20 \mathrm{cm}$ is placed in contact with a concave lens of focal length $10 \mathrm{cm}$. What is the power of the combination?

For the reflected ray, shown in the diagram draw the corresponding incident ray.

Define $1$ dioptre of power of a lens.

Give an application of diffused light.

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.