Question

It takes 4 years for light to travel from a galaxy to the Earth. Given the speed of light as $3\times {10}^8$ m/s, what is the distance of the galaxy from the Earth? [2 marks]

Answer 1

Given speed of light = $3\times {10}^8$ m/s
Time taken for the light to travel from the galaxy to the Earth is 4 years.
4 years $=4\times 365$ days
4 years $=4\times 365\times 24$ hours
4 years $=4\times 365\times 24\times 60\times 60$ seconds [1 mark]
4 years = 126144000 seconds = $1.26\times {10}^8$ seconds
distance = speed $\times$ time = $3\times {10}^8\times 1.26\times {10}^8$ metres
distance $=3.78\times {10}^{16}$ metres [1 mark]