Special Relativity explains the Doppler Effect of light (electromagnetic radiation). When the radiation source approaches (or recedes from) the observer, the radiation frequency is shifted. See Section 7 of Einstein's 1905 paper On the Electrodynamics of Moving Bodies. Enter below: (1) the speed of the source toward (positive) or away from (negative) the observer as a fraction of the speed of light and (2) the wavelength of light in nanometers in the range of 380 - 780. This is visible light. Click "Show Colors" to see the colors of the light in the source and observer frames of reference in the two boxes beneath the input values.
Speed Light Wavelength (nm) Source Observer