# What happens when light moves perpendicular to a moving object?

Imagine the folllowing situation: A coherent light source is attached to a car such that the emitted light beam path is "being crossed over" by the car i.e. the long parallel light beams are struck by the moving car such that their path wrt exach other is a 'X'. Say the car is going at a speed of 3000m//s and the coherent light source is attached to the car. do the light particles experience doppler shift? do they photon get absored by th car body? does the path of the photons change in any velcoity due to a gain in momentum? what about the angle of being hit - does it influence any of the above properties of light particles?

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Your question is not clear. If the light source is attached to the car, how will the car cross the beam? It will be always a delta(t) behind the beam and cannot be hit by it or hit it. –  anna v Jun 13 '11 at 14:36

Specifically, the energy will be increased by a gamma factor ($E'=\gamma E$) and the angle of attack will be
$$\tan\theta'=\frac{1}{\gamma \beta}$$
where $\gamma$ and $\beta$ are the parameters for the frame of the moving car. Of course the speed of the photons is always the speed of light $c$.