Question

A laser beam is sent to the moon and reflected back to earth by a mirror placed on the moon by an astronaut. If the moon is at $3,84,000 \, km$ distance from earth, how long does it take the light to make the round trip?

• A. $5min$
• B. $2.5min$
• C. $2.5\text{s}$
• D. $500\text{s}$

Answer 1

Answer: (C)

Given:
The distance of the moon $\left(\right. s \left.\right) = 384000 \, \text{km}$
$= 384000 \times 10^{3} \, \text{m}$
We known speed of light $\left(\right. c \left.\right) = 3 \times \left(10\right)^{8} \, \text{m/s}$
Total time taken by the laser beam to make the round trip is
$t=\frac{2 s}{c}=\frac{2 \times 384000 \times 1000}{3 \times 10^{8}}=2.5 \, s$