A star is \(8.1 \times 10^{13}\) km away from the earth. Suppose light travels at a speed of \(3 \times 10^5\) km/s. How long will it take light from the star to reach the earth?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(7.5 \times 10^4\) hrs.
Light travels at a steady speed, so the time = distance ÷ speed. The answer in seconds is then converted to hours, as 1 hour = 3600 seconds.
Time \(\displaystyle =\frac{8.1\times10^{13}}{3\times10^{5}}=2.7\times10^{8}\) s
In hours: \(\displaystyle \frac{2.7\times10^{8}}{3600}=\frac{2.7\times10^{8}}{3.6\times10^{3}}=0.75\times10^{5}=7.5\times10^{4}\) h
Options (3) and (4) are in seconds but do not match \(2.7\times10^{8}\) s.