Question

A light pulse is emitted from a space ship travelling at a speed of $c/2$ towards the planet. The distance between the spaceship and the planet is $4.5\times {10}^{8}m$. Find the time required for the light pulse to reach the planet.

• A. $0.5sec$
• B. $1.0sec$
• C. $1.5sec$
• D. $2.0sec$
• E. $2.5sec$

Answer 1

The correct option is C $1.5sec$

From the special theory of relativity, the speed of light remains the same in all inertial frames, that is, frames in which bodies are not accelerating.
Thus the speed of light pulse as observed by person on planet=$c=3\times {10}^{8}m/s$
Hence time taken for pulse to reach planet=$\frac{d}{c}=\frac{4.5\times {10}^{8}m}{3\times {10}^{8}m/s}=1.5s$