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I have a question after I read the second postulate of relativity. Is the velocity of light is constant for moving bodies also? suppose, a body at rest emit a photon from its frame of reference and another body is moving with half the velocity of light observes the velocity of photon from its frame of reference. So, my question is that will the velocity of light will be the same if body moving with half the speed of light observes from its frame of reference? Thank you on advance.

Piyush Kapoor
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Yes. The speed of the photon will appear the same to all observers.

ohwilleke
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Imagine that there was a large wall, wall#1, and that this wall was covered with LED's (Light Emitting Diodes). Off to the right, at a far distance of 300,000 km away from this wall, is another wall, wall#2. Also, Wall#1 and wall#2 are of equal size.

Now if all of the LED's at wall#1 simultaneously emitted a pulse of light, the pulse of light, released from any one LED of the collection of LED's, will have reached an equal position on wall#2, just one second later. Thus light will have traveled 300,000 km in 1 second, hence the speed of light.

This releasing of short pulses of light can be set to repeat at say 1 second intervals. The results for each pulse will be the same. Each pulse of light will reach an equal position on the destination wall in 1 second.

If any measuring equipment that is designed to measure the speed of light was positioned between the two walls, it too would measure the speed of any light pulse that it is exposed to as being 300,000 km/s. This will occur no matter where the measuring equipment is positioned.

Even if the measuring equipment was in motion to the right toward wall#2, it would still measure the speed of any light pulse that it is exposed to as being 300,000 km/s. If the velocity of the movement of the measuring equipment is changed to a new velocity, say half the speed of light for instance, the results of measuring the traveling speed of these pulses of light, will remain the same.

Even if the measuring equipment was in motion to the left toward wall#1, it would still measure the speed of any light pulse that it is exposed to as being 300,000 km/s. If the velocity of the movement of the measuring equipment is changed to a new velocity, again say half the speed of light for instance, the results of measuring the traveling speed of these pulses of light, will also remain the same.

Sean
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