State and define the S.I. unit of power of a lens.

A 5 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 18 cm at a distance of 12 cm from it. Use lens formula to determine the position, size and nature to the image formed.

What is meant by ‘power of a lens’?

A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close focal contact with each other. Calculate the lens power of this combination.

With the help of a ray diagram, state the meaning of refraction of light. State Snell’s law of refraction of light and also express it mathematically.