Using What You’ve Learned About Electromagnetic Radiation and Sound

(Acoustic Energy) to Measure Speed and Distance

April 2007

What We Know About Sound

Sound waves coming from an object travel from the source of the sound to the observer.

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The distance between crests is known as the wavelength.

What We Know About Sound

Sound waves from a moving object are compressed in front of the object.

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The wavelength is shorter.

Shorter wavelength = higher pitch.

What We Know About Sound

Sound waves from a moving object are stretched out behind the object.

]The wavelength is longer.

Longer wavelength = lower pitch.

What About Other Types of Waves?

Question: Is there anything unique about sound waves, or would any type of wave be compressed/expanded by an object’s motion?

Answer: Any kind of wave is affected by motion. Radio waves and even visible light waves are affected!

The effect of motion on a wave is called the Doppler Effect.

What About an Echo?

Question: What would happen if a sound were reflected by the moving object?

Answer: The reflected sound waves are compressed by the object’s motion. The echo comes back at a higher pitch!

What Can We Do With This Information?Question: If we know the pitch (or wavelength) of a wave that we sent, and we know the pitch (or wavelength) that we got back as an echo, what do we know?

Answer: We can tell whether an object is standing still, moving toward us, or moving away from us. By doing some math, we can also tell how fast it’s moving!

How Police Radar Works

Instead of sending sound waves, a police radar unit sends radio waves in the direction of a moving object.

By measuring how much the wavelength (frequency) has changed when it’s reflected by the moving vehicle, the radar unit can compute the vehicle’s speed.

This technique is called “Doppler Radar.”

Some Cool Facts About Radar

Radar can be (and is) used for much more than just enforcing the speed limit!

Weather forecasters have used radar (RAdio Detection And Ranging) since the 1940s to detect precipitation, especially thunderstorms.

Doppler radar systems can detect the location of precipitation areas as well as motions toward or away from the radar.

How Else Can We Measure Speed?

By sending a signal and measuring how long it takes for the echo to come back, we can measure distance.

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By sending a second signal and measuring how long it takes for the echo to come back, we can measure distance once again.

By keeping track of the time between the first and second signals, we can determine how fast the object is moving.

LIDAR Speed Measurement

A pulse of invisible infrared laser light is aimed at the vehicle. The reflection comes back at the speed of light (about one billionth of a second per foot)!

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Another pulse of light is sent, and its reflection time is measured just like the first pulse.

LIDAR (LIght Detection And Ranging) units use the difference between these readings to compute the speed of the vehicle.