Suppose a rocketship is receding from the earth at a speed of 2/10th the velocity of light. A light in the rocketship appears blue to the passengers on the ship. What colour would it appear to an observer on the earth?

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Submitted by examrobotsa on

Q: 111 (IAS/1995)
Suppose a rocketship is receding from the earth at a speed of 2/10th the velocity of light. A light in the rocketship appears blue to the passengers on the ship. What colour would it appear to an observer on the earth?

question_subject: 

Science

question_exam: 

IAS

stats: 

0,23,51,25,14,12,23

keywords: 

{'blue': [0, 1, 0, 0], 'colour': [11, 6, 13, 28], 'yellow': [1, 1, 4, 3], 'velocity': [0, 2, 2, 6], 'observer': [0, 1, 2, 3], 'light': [16, 4, 34, 62], 'rocketship': [0, 1, 0, 0], 'orange': [2, 1, 1, 4], 'passengers': [0, 0, 1, 0], 'speed': [0, 1, 2, 0], 'ship': [3, 2, 2, 5], 'earth': [0, 1, 1, 1]}

When light moving away from an observer its frequency or wavelength changes due to the Doppler effect. In this example, light is emitted from a source moving away (the rocket) at a significant fraction of the speed of light. This causes the light waves to `stretch out` or redshift, resulting in the observer seeing a longer wavelength (lower frequency) of light than the source actually emitted.

Blue light has shorter wavelengths while yellow-orange light has longer wavelengths. Hence, the light appears blue to passengers on the ship (where the source is stationary with respect to them) and yellow-orange to an observer on the earth (from whom the source is receding).

Option 1 - Blue, might seem plausible as it`s the colour seen by the passengers. However, the observer`s viewpoint is different.

Option 2 and 3 - Orange and Yellow, are closer to the correct answer as the colour seen should be a longer wavelength than blue.

Option 4 - Yellow-orange, is correct as it`s a blend of two colours that have longer wavelengths than blue, rightly suggesting a phenomenon of redshift.