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