When the fire truck passes, the sound of its siren changes. The sound waves emitted by the approaching fire truck are pushed closer together by its motion, so the notes seem to have a higher frequency than they'd have if you and the fire truck were both parked. In the same way, the sound waves from a receding fire truck are appear to have a lower frequency. The Doppler effect is often applied with laser beams. By reflecting a laser off an object and noting the shift in frequency, the speed of the object, such as a speeding car, can be derived. The cop waiting by the roadside with his speed gun uses the same physical effect; he hounces a radar wave off your car and meausres the frequency of the reflected wave. The frequency will be higher the faster you're going. Better not be going too fast, because the policeman can measure your speed to a fraction of a mile per hour using this method.

Another way radar can be used in conjunction with the Doppler effect allows scientist to "see" objects using radar. Because objects have shape, lasers pointing at them will hit at different times on the same object. Scientists note the different Doppler effects caused by different areas of an object. Using this data, a model or image of the object can be created.

Using Doppler Effect, we can measure the distance or the velocity of an object in the space too. Scientists can accurately determine the distance to Moon (within an error of 3 cm) by measuring the time delay required for a radio signal to return to the earth. At 240,000 miles, the Moon is close enough for us to make active measurements. But for most of astronomy, the situation is different; we can't experiment with a star in another galaxy yet (because the power needed would be too great and the time it would take for the wave to bounce back is too long).