What is total internal reflection?

What is critical angle? How is it related to the refractive index?

A convex lens forms a real and inverted image of a needle at a distance of $40 \mathrm{cm}$ from it. Where is the needle placed in front of the convex lens if the size of the image is equal to that of the object? Also, find the power of the lens.

Find the power (in dioptre) of a concave lens of focal length $50 \mathrm{cm}$.

When an object is placed before a lens the lens forms a virtual image for any position of the object. Is the lens convex or concave?

Name four common defects of vision. Give the causes and remedial measure.