We know the Doppler Effect as the reason why horns and sirens drop in tone as they rush by us. But the Doppler Effect works on objects that are twisting around, too. We'll tell you how the Doppler Effect gets in a tangle.

The Doppler Effect and Sound

The basic Doppler Effect is well-known to anyone who listens to the traffic driving by them. As the cars zoom by, the sound of their engines, and their horns, drops in pitch. We determine the pitch of a sound by the frequency of sound waves. The more "peaks," or high air density areas, that hit our ears within a second, the higher the pitch.


The horn of a car at rest puts out a certain number of waves per second, and those waves spread uniformly through the air. When a car is coming towards you, the waves in front of the car are crushed together, like the waves in front of the bow of a ship. The number of waves per second, the frequency, increases and you hear a high note. When the car passes you, the waves spread out behind it, with space between each wave. The frequency decreases and the note drops.

The Doppler Effect and Light

The same effect works with light. An object at rest will emit a certain frequency of light. When the object comes towards the observer, the peaks of light cram together and create, for the still observer, a higher frequency. As visible light increases in frequency, it moves towards the blue end of the visible spectrum, and so we call this change in frequency blueshift (even if it occurs in the frequencies of light that are invisible to the human eye). As the object moves away, the frequency of the light waves it emits decreases, which we call redshift.

This is how radar and lidar guns work. The gun emits a frequency of light. The light bounces off a car and returns to the gun. If the light bounces off an object moving towards the gun, the frequency of the returning light will increase. The faster the car is going, the more the frequency will increase.

The Rotational Doppler Effect

But what if the car isn't speeding towards the radar gun? What if it's spinning in place? For a time, it didn't look like we would be able to detect kind of motion through the Doppler Effect. Then we learned something about how the polarization of light changes it frequency.

Light waves can be viewed as vibration, like the waves on a piece of string being shaken up and down. When light is polarized, all the light is vibrating in one direction. Light can also be circularly polarized. (Imagine that string being drawn in a circle like a jump rope and the motion traveling down its length.) When light is circularly polarized, it twists in a sort of corkscrew as it travels.

In 1981, a chemist discovered that when light is circularly polarized, the polarization changes its frequency. What's more, if the light then hits a spinning object, a rotating reflective surface or a spinning gas cloud, the light's polarization and frequency changes again.

Redshift and blueshift don't have to just tell us about how objects are moving toward us or away from us. They might tell us how the objects are twisting around.

Top Image: Aloe Plant

Via Physics Central, Science Now, Nature.