Question Constellation Sign up for free AQA · GCSE · Physics · Higher
Light Travel Time from Arcturus
The distance between Arcturus and the Earth is 3.6 × 10¹⁴ km. speed of light = 3.0 × 10⁸ m/s. Use the Physics Equations Sheet.
Calculate the time taken for light from Arcturus to reach the Earth.
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Marking points
- 1 Converts kilometres to metres correctly.
- 2 Substitutes distance and light speed into distance = speed × time.
- 3 Rearranges correctly for time.
- 4 Gives 1.2 × 10⁹ s, with official consequential marking preserved.
Full-mark answer
3.6 × 10¹⁴ km = 3.6 × 10¹⁷ m. Using s = vt, t = s/v = (3.6 × 10¹⁷) ÷ (3.0 × 10⁸) = 1.2 × 10⁹ s.
Why this answer loses marks
Divides the stated distance by the light speed immediately.
The distance and speed use different length units, so the distance must be converted before applying the equation.