Question

The diameter of the moon is about 3500 km. What will be the diameter of the image formed by a concave mirror of radius 3 m? Assume that the distance of the moon is about $3\cdot 5\times 105km.$

• A. 5 mm
• B. 10 mm
• C. 15 mm
• D. 20 mm

Answer 1

The correct option is C 15 mm

Here, angle subtended by moon
$=\frac{3500}{3\cdot 5\times {10}^5}radians$
Also, focal length of the mirror,
$f=R/2=1\cdot 5m$
$=1\cdot 5\times {10}^{3}mm$
Required diameter of image

$\therefore D=f\theta =1\cdot 5\times {10}^3\times \frac{3500}{3\cdot 5\times {10}^5}$
$=15mm$