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

From `(1)/(v) + (1)/(u) = (1)/(f)`
`(1)/(v) + (1)/(oo) = (1)/(f)`
`:. v = f`
That is, image is formed at the focus F of the concave mirror. If A'B' is the size/diameter of image
`theta = (A'B')/(PF) = (d)/(f)`
`d = f theta`