What is the $SI$ unit of the power of a lens?

State the laws of reflection.

What can be measured using a dioptre meter?

In which medium is the speed of light the highest?

What is the minimum number of rays required for locating the image formed by a concave mirror for an object? Draw a ray diagram to show the formation of a virtual image by a concave mirror.

Two lenses of power $+3.5 D$ and $-2.5 D$ are placed in contact. Find the power and focal length of the lens combination.