The sun (diameter `d`) subtends an angle `theta` radian at the pole of a concave mirror of focal length `f`. What is the diameter of the image of the sun formed by the mirror ?

Explanation

Since the sun is at large distance , u is very large and so `(1)/(u) approx 0`
`therefore (1)/(v) + (1)/(u) = (1)/(f) implies (1)/(v) = (1)/(f) implies v = -f`

The image of sun will be formed at the focus and will be real, inverted and diminished
`A'B'` = height of image and `theta = ("Arc")/(Radius") = (A'B')/(FP) implies theta = (d)/(f) implies d = ftheta`