Question

Two stars situated at distances of 1 and 10 light years respectively from the earth appear to possess the same brightness. The ratio of their real brightness is

• A. 1 : 10
• B. 10 : 1
• C. 1 : 100
• D. 100 : 1

Answer 1

Answer: (C)

1 : 100

Answer 2

$I = \frac{L}{r^{2}} \Rightarrow \frac{L_{1}}{r_{1}^{2}} = \frac{L_{2}}{r_{2}^{2}}$ (I is same)

$\Rightarrow \frac{L_{1}}{L_{2}} = \frac{r_{1}^{2}}{r_{2}^{2}} = \left( \frac{1}{10} \right)^{2} = 1:100$.