http://www.einstein-online.info/spotlights/doppler Albert Einstein Institute: "The frequency of a wave-like signal - such as sound or light - depends on the movement of the sender and of the receiver. This is known as the Doppler effect. (...) In the above paragraphs, we have only considered moving sources. In fact, a closer look at cases where it is the receiver that is in motion will show that this kind of motion leads to a very similar kind of Doppler effect. Here is an animation of the receiver moving towards the source: (...) By observing the two indicator lights, you can see for yourself that, once more, there is a blue-shift - the pulse frequency measured at the receiver is somewhat higher than the frequency with which the pulses are sent out. This time, the distances between subsequent pulses are not affected, but still there is a frequency shift: As the receiver moves towards each pulse, the time until pulse and receiver meet up is shortened. In this particular animation, which has the receiver moving towards the source at one third the speed of the pulses themselves, four pulses are received in the time it takes the source to emit three pulses."
(distance between subsequent pulses not affected)/(time until pulse and receiver meet up shortened) = (speed of pulses relative to receiver increased)
L/t' = c' > L/t = c
where t is the time until pulse and receiver meet up when the receiver is stationary, t' is the shortened time when the receiver is moving, L is the wavelength, c is the speed of the light relative to the stationary receiver and c' is the speed of the light relative to the moving receiver.