Question

The Sun is 150 x 106 km away. If light travels at 3 x 108 m/s, how long does it take light from the Sun to reach the Earth?

time = \frac{distance}{speed}

distance = 150 x 106 km = 150 x 106 x 1000 m = 150 x 109 m

speed = 3 x 108 m/s

time = \frac{{150}\times{10}^9{m}}{{3}\times{10}^8{m/s}}

time = 500 s = 8.3 minutes

It takes 8.3 minutes for light to reach the Earth from the Sun.

Question

A galaxy is observed to be 2.4x1020 m away from Earth?

  1. How far is that in light years?
  2. How long would it take for light to travel from the galaxy to earth?

1. 1 light year = 9.46 x 1015 m

The number of light years = \frac{{2.4}\times{10}^20{m}}{{9.46}\times{10}^{15}{m}}

The number of light years = 25,370.

The galaxy is 25,370 light years away.

2. It would take 25,370 years for light to reach the earth from the galaxy.

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