In 1842, Austrian physician and astronomer Christian Doppler ascertained that wavelength λ adopted by observer is related to length of radiation source wave by ratio:
Where V is projection of source speed on vision light. The law he had discovered was named Doppler Law:
Bias of lines in star spectrum with respect to comparison spectrum to the red side means that the star is leaving us. Bias to the violet side means that the star is approaching lines. Since the Earth rotates around the Sun with the speed V = 30 km/s, lines in spectrums of stars moving away from the Earth are shifted to red side by Δλ/λ = V/c = 10-4. This bias will be equal to 0.05 nm (0.5 Å) for the line λ = 500 nm. Lines of stars approaching the Earth will be shifted by the same value to the violet side.
The model to the right shows the diagram of experiment demonstrating the effect. The star moves relatively to the Earth with velocity ν0 at the angle φ with direction to the Earth. Velocity value and direction may be changed with the aid of appropriate input boxes. Star ray velocity ν1 (that is, projections of velocity to Earth direction) is reflected in message box.
Star spectrum and hydrogen laboratory spectrum for comparison occupies central part of the model. Since total length of spectrum optical area is considerably larger than spectral lines' bias because of Doppler effect, the screen shows only part of spectrum corresponding to certain bright line of hydrogen spectrum. Length of spectral line may be changed by selecting it from WaveLength list.