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## Main Question or Discussion Point

Consider frame S1 and another frame S2 moving with a velocity v with respect to S1 along +X direction. Consider a photon comes in -X direction. Velocity of photon in S1 is c.I would like to find the velocity of light in S2 from S1 without forgetting length contraction and time dilation as follows.

So from S1 point of view using relativity theory the velocity of photon measured by S2 will be

What is wrong here?

**Observer in S1 thinks like this. For one second in his clock the clock in S2 will run [tex]\sqrt{1-v^{2}/c^{2}}[/tex] seconds. In this time the relative separation between the observer in S2 and photon decreases by (c+v)[tex]\sqrt{1-v^{2}/c^{2}}[/tex] considering length contraction.**So from S1 point of view using relativity theory the velocity of photon measured by S2 will be

**distance/ time = c+v**. But it has to be c which we get from velocity addition rule also.What is wrong here?