An observer is moving in space toward a distant star at 200 km/s while the star is moving toward the observer at 400 km/s; the relative velocity being 600 km/s of approach. what relative change in frequency of the light from the star as seen by the observer? (the speed of light in space is 3.00 ´ 105 km/s).

The correct answer is: 0.2% (decrease)

Explanation:

The observed frequency can be found by using the following equation:
 --- (1)

Where  = Speed of the observer = 200 *1000 m/s
V = speed of the star = 400 *1000 m/s

Plug in the values in (1):
(1) => 

--- (2)

Change in frequency is given as: % --- (3)

Put (2) in (3):
%

=> -0.2%

Negative sign shows that it decreases!

Hence it is 0.2% (decrease).

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