What do you understand by the term magnification? Write an expression for it for a lens, explaining the meaning of the symbols used.

Define the term power of a lens. In what unit is it expressed ?

A lens of power +2.5D is kept in contact with another lens of power -2.5D. What will be the power of the combination of the two lenses? How will the combination behave?

(a) At what position a candle of length 3 cm be placed in front of a convex lens so that its image of length 6 cm be obtained on a screen placed at a distance 30 cm behind the lens?

(b) What is the focal length of the lens in part (a)?