Question

A boy is trying to start a fire by focusing sunlight on a piece of paper using an equiconvex lens of focal length 10 cm. The diameter of the sun is $ 1.39 \times 10^9 $ m and its mean distance from the earth is $ 1.5 \times 10^{11} m.$ What is the diameter of the sun's image on the paper?

• A. $ 6.5 x 10^ 5m $
• B. $12.4 \times 10^{-4} m $
• C. $ 9.2 x 10^ 4m $
• D. $ 6.5 x 10^ 4m$

Answer 1

Answer: (C)

$\frac{size of image}{size of object } = | \frac{v}{u}|$
$\Rightarrow size of the image = \frac{ 1.39 \times {10}^9 \times {10}^{-1}}{ 1.5 \times {10}^{11}} = 0.92 \times {10}^{-3} m $
$ = 0.92 \times {10} ^{-3 } m $
size of the image $ = 9.2 \times ^{-4} m $