Question

The sun (diameter D) subtends an angle of θ radians at the pole of a concave mirror of focal length f. The diameter of the image of the sun formed by the mirror is

• A. fθ
• B. $\frac { f 2 \theta } { D }$
• C. 2 fθ
• D. Dθ

Answer 1

Answer: (A)

fθ

Answer 2

Since the sun is at very large distance, p = ∞

⇒ $\frac { 1 } { \infty } + \frac { 1 } { q } = \frac { 1 } { f }$

⇒ q = f

If the diameter of the image be d, ⇒d = (2α) q Putting 2 α = θ and q = f we obtain, d = f θ