9. A light-nanosecond is the distance that a photon can travel at the speed of light in one billionth of a second. The speed of light is $3 \times 10^{8} \mathrm{~ms}^{-1}$.
How far is a light-nanosecond?
A $3 \mathrm{~cm}$
B $30 \mathrm{~cm}$
C $3 \mathrm{~m}$
D $30 \mathrm{~m}$
E $300 \mathrm{~m}$
Solution
B

One billionth of a second is $\frac{1}{10^{9}}$ seconds.
Hence, in one billionth of a second light travels
$$
\frac{1}{10^{9}} \times\left(3 \times 10^{8}\right) \mathrm{m}=\frac{3}{10} \mathrm{~m}=30 \mathrm{~cm} \text {. }
$$

Therefore a light-nanosecond is $30 \mathrm{~cm}$.
For INVESTIGATION
9.1 A light-minute is the distance that a photon travels in one minute at the speed of light. The mean distance of the Earth from the Sun is approximately 150 million kilometres. How many light-minutes is that?