(i) Define power of a lens and write its SI unit. (ii) A convex lens of power 4D is placed at a distance of 40 cm from a wall. At what distance from the lens should a candle be placed so that its image is formed on the wall?

(i) Define power of a lens and write its SI unit. (ii) A convex lens

1.	(i) Define power of a lens and write its SI unit. (ii) A convex lens of power 4D is placed at a distance of 40 cm from a wall. At what distance from the lens should a candle be placed so that its image is formed on the wall?
Answer» Solution :(i) Reciprocal of focal length expressed in metres is called power. SI UNIT is dioptre. (ii) `F=(1)/(P)=(1)/(4D)=(1)/(4)m=25cm` `v=40m`. `(1)/(v)-(1)/(u)=(1)/(f)` `(1)/(u)=(1)/(v)-(1)/(f)=(-3)/(200)` `"or"u=(-200)/(3)cm` So candal should be placed 200/3 cm from the LENS.