What's Doppler Effect?

The Doppler Effect was named after Christian Doppler, who came up with the idea in 1842. He thought of the frequency of sound waves when the source or the observer were moving. He said that if one of them were approaching, the frequency of the pitch would be higher; and if they were moving away from, the frequency of the pitch would be lower. As we know there are many examples of this in everyday life. For example, train whistles, sirens, and even cars (example: carhorn). In all of these examples you will notice a change in the sound when the object is moving closer or further away from you.

Furthermore, the Doppler Effect does not only apply to sound waves. Although first discovered for sound waves, the Doppler Effect holds true for all types of waves including light. 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. Edwin Hubble used the Doppler Effect to determine that the universe expands. Hubble found out the light between the galaxies shift toward higher frequencies or the red end of the spectrum. This is known as a red Doppler shift, or the red-shift. If the galaxies were moving toward him then the light observed would be blue-shifted.

An example of the Doppler Effect is:
When a cop uses his radar gun, the motion waves from the car emit a speed that comes up on his gun and if it is higher than the speed limit you will be pulled over. Another example of this effect is when a fire truck approaching you, the noise of the siren becomes higher and higher, and when it is moving away the sound gets lower and lower.