A point source of light is kept at a distance of 15 cm from a converging lens, on its optical axis. The focal length of the lens is 10 cm and its diameter is 3 cm. A screen is placed on the other side of the lens , perpendicular to the axis of the lens, at a distance 20 cm . from it. Then find the area of the illuminated part of the screen ?

Answer

`(pi)/(4) cm^(2)`

Explanation

`v = (uf)/(u+f) = (-15 xx 10)/(-15 +10) = 30`
`triangleABC` & `triangleA'B'C` are similar `(x)/(10) = (1.5)/(30) implies x = (1)/(2)`
`therefore` Area = `pix^(2) = pi xx (1)/(4) = (pi)/(4) cm^(2)`