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 1.39×109 m and its mean distance from the earth is 1.5×1011 m. What is the diameter of the Sun’s image on the paper ?

Answer 1

wkt,

magnification = image height/obj. height = image dist./ obj. dist.

therefore,   I/O = v/u       ;  I = image height( here diameter of image)

                                           O = object height ( here diameter of obj.)

                                           v = image distance (here focal length) = 10 cm

                                           u = obj. distance

placing the values from ques.,

I = v*O/u

 = 10 * 10^(-2) * 1.39 *10^9/ 1.5 * 10^11

 =  0.92 * 10^ (-3) m

 = 9.2 * 10^ (-4) m