An electric bulb, connected to a make and break power supply, switches off and on every second in its rest frame. What is the frequency of its switching off and on as seen from a spaceship travelling at a speed 0.8c?

Explanation

Given ` v = 0.8 c `
` f' = f (sqrt 1 - v^(2) / c^(2))`
` f' = (sqrt 1-0.64 c^(2) / c^(2)) `
= (sqrt 0.36) = 6 s^(-1) ` .