Question

A new unit of length is chosen such that the speed of light in vacuum is unity. What is the distance between the sun and the earth in terms of the new unit if light takes 8 min and 20 s to cover this distance?

• A. 300
• B. 400
• C. 500
• D. 600

Answer 1

Answer: (C)

500

Answer 2

Here,
Speed of light in vacuum,
$c = 1$ new unit of length $s^{-1}$
Time taken by light of sun to reach the earth
$t = 8$ min and $20$ seconds
$= (8 ? 60 + 20) s = 500\, s$
$\therefore\quad$ Distance between the sun and the earth
$S = ct = 1$ new unit of length $s^{-1} ? 500 \,s$
$= 500$ new unit of length