When an object emits light or sound, that light or sound will have a particular wavelength. If an object is moving, the object's motion will cause a distortion in the wavelength pattern relative to a "stationary" sensor. Keep in mind that this is an exercise in relativity. From the moving object's perspective, there is no distortion of the wavelength of light. It is sensors that are not moving at the same velocity as the object that will detect the shift in wavelength. Here is an analogy to clarify: Suppose a baseball player who is running at 10 mph throws a baseball at 50 mph in the same direction he is running. Relative to the running baseball player the ball is moving away at 50 mph. However, a stationary baseball player that catches the throw detects the ball approaching at 60 mph. This is because the motion of the running baseball player and the motion of the thrown ball are added together. An object moving toward a sensor will have a more rapid forward displacement of wave energy relative to the sensor. The sensor will detect a higher pitch or relatively shorter wavelength light. An object moving away from a sensor will have a less rapid forward displacement of wave energy relative to the sensor. The sensor will detect a lower pitch or relatively longer wavelength light. |