The sudden change in pitch of a car horn as a car PASSES by (source motion) or in the pitch of a boom box on the sidewalk as you drive by in your car (observer motion) was first explained in 1842 by Christian Doppler. His Doppler EFFECT is the shift in FREQUENCY and wavelength of waves which results from a source moving with RESPECT to the medium, a receiver moving with respect to the medium, or even a moving medium.

Although first discovered for sound waves, the Doppler effect HOLDS true for all types of waves including light (and other electromagnetic waves). The Doppler Effect for light waves is usually described in terms of colors rather than frequency. A red shift occurs when the source and observer are moving away from each other, and a blue shift occurs when the source and observer are moving towards each other. The red shift of light from remote galaxies is proof that the universe is expanding.

The sudden change in pitch of a car horn as a car passes by (source motion) or in the pitch of a boom box on the sidewalk as you drive by in your car (observer motion) was first explained in 1842 by Christian Doppler. His Doppler Effect is the shift in frequency and wavelength of waves which results from a source moving with respect to the medium, a receiver moving with respect to the medium, or even a moving medium.

Although first discovered for sound waves, the Doppler effect holds true for all types of waves including light (and other electromagnetic waves). The Doppler Effect for light waves is usually described in terms of colors rather than frequency. A red shift occurs when the source and observer are moving away from each other, and a blue shift occurs when the source and observer are moving towards each other. The red shift of light from remote galaxies is proof that the universe is expanding.