not as fast as yellow!
You yellow guys really don't give up. One more time...
In classical physics, where the speeds of source and the receiver relative to the medium are lower than the velocity of waves in the medium, the relationship between observed frequency f and emitted frequency f0 is given by:[3]
f = \left( \frac{c + v_r}{c + v_{s}} \right) f_0 \,
where
c \; is the velocity of waves in the medium
v_{r} \, is the velocity of the receiver relative to the medium; positive if the receiver is moving towards the source.
v_{s} \, is the velocity of the source relative to the medium; positive if the source is moving away from the receiver.
The frequency is decreased if either is moving away from the other.
The above formula assumes that the source is either directly approaching or receding from the observer. If the source approaches the observer at an angle (but still with a constant velocity), the observed frequency that is first seen is higher than the object's emitted frequency. Thereafter, there is a monotonic decrease in the observed frequency as it gets closer to the observer, through equality when it is closest to the observer, and a continued monotonic decrease as it recedes from the observer. When the observer is very close to the path of the object, the transition from high to low frequency is very abrupt. When the observer is far from the path of the object, the transition from high to low frequency is gradual.
In the limit where the speed of the wave is much greater than the relative speed of the source and observer (this is often the case with electromagnetic waves, e.g. light), the relationship between observed frequency f and emitted frequency f0 is given by:[3]
Observed frequency Change in frequency
f=\left(1-\frac{v_{s,r}}{c}\right)f_0
\Delta f=-\frac{v_{s,r}}{c}f_0=-\frac{v_{s,r}}{\lambda_{0}}
where
v_{s,r} = v_s - v_r \, is the velocity of the source relative to the receiver: it is positive when the source and the receiver are moving further apart.
c \, is the speed of wave (e.g. 3×108 m/s for electromagnetic waves travelling in a vacuum)
\lambda_{0} \, is the wavelength of the transmitted wave in the reference frame of the source.
These two equations are only accurate to a first order approximation. However, they work reasonably well when the speed between the source and receiver is slow relative to the speed of the waves involved and the distance between the source and receiver is large relative to the wavelength of the waves. If either of these two approximations are violated, the formulae are no longer accurate.
The Doppler effect for electromagnetic waves such as light is of great use in astronomy and results in either a so-called red shift or blue shift. It has been used to measure the speed at which stars and galaxies are approaching or receding from us, that is, the radial velocity. This is used to detect if an apparently single star is, in reality, a close binary and even to measure the rotational speed of stars and galaxies.
The use of the Doppler effect for light in astronomy depends on our knowledge that the spectra of stars are not continuous. They exhibit absorption lines at well defined frequencies that are correlated with the energies required to excite electrons in various elements from one level to another. The Doppler effect is recognizable in the fact that the absorption lines are not always at the frequencies that are obtained from the spectrum of a stationary light source. Since blue light has a higher frequency than red light, the spectral lines of an approaching astronomical light source exhibit a blue shift and those of a receding astronomical light source exhibit a redshift.
Bottom line the faster an object approaches the bluer it appears. The faster an object recedes the more red it appears. Morale of the story is that fast cars are blue when approaching and red when receding, THEY ARE NEVER YELLOW.