The sun subtands an angle radians 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 sum is very distance u is very large and so `(1)/(u)` is practially zero
`(1)/(u) ~~ 0 :. , (1)/(u) + (1 )/(V ) = (1)/(f ) rArr (1)/( v ) = (1)/(f ) rArr v= - f`

The image of sum will be formed at the focus and will be real inverted diminished
AB = height of image
`theta = (Arc )/( Radius) = (AB)/(FP) rArr theta = ( d)/( f) rArr d= ftheta`