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

For the sun, `u = oo, v = ?` diameter of image, `d = A' B' = ?`, Fig.
From `(1)/(v)+(1)/(u)=(1)/(f)`
`(1)/(v)+(1)/(oo)=(1)/(f) :. v = f`
i.e., image is formed at the focus `F` of concave mirror. If `A' B'` is the size/diameter of the image of the sun, then from
`theta = (A' B')/(PF) = (d)/(f)`
`d = theta f`.
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