<P> Note that redshift is also used to measure the expansion of space, but that this is not truly a Doppler effect . Rather, redshifting due to the expansion of space is known as cosmological redshift, which can be derived purely from the Robertson - Walker metric under the formalism of General Relativity . Having said this, it also happens that there are detectable Doppler effects on cosmological scales, which, if incorrectly interpreted as cosmological in origin, lead to the observation of redshift - space distortions . </P> <P> The use of the Doppler effect for light in astronomy depends on our knowledge that the spectra of stars are not homogeneous . 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 blueshift and those of a receding astronomical light source exhibit a redshift . </P> <P> Among the nearby stars, the largest radial velocities with respect to the Sun are + 308 km / s (BD - 15 ° 4041, also known as LHS 52, 81.7 light - years away) and - 260 km / s (Woolley 9722, also known as Wolf 1106 and LHS 64, 78.2 light - years away). Positive radial velocity means the star is receding from the Sun, negative that it is approaching . </P> <P> The Doppler effect is used in some types of radar, to measure the velocity of detected objects . A radar beam is fired at a moving target--e.g. a motor car, as police use radar to detect speeding motorists--as it approaches or recedes from the radar source . Each successive radar wave has to travel farther to reach the car, before being reflected and re-detected near the source . As each wave has to move farther, the gap between each wave increases, increasing the wavelength . In some situations, the radar beam is fired at the moving car as it approaches, in which case each successive wave travels a lesser distance, decreasing the wavelength . In either situation, calculations from the Doppler effect accurately determine the car's velocity . Moreover, the proximity fuze, developed during World War II, relies upon Doppler radar to detonate explosives at the correct time, height, distance, etc . </P>

Which situation is the best analogy for the doppler effect