The change in electromagnetic frequency that occurs when the source of the radiation and its observer move toward or away from each other. The faster they come together, the higher the frequency. The ...

The headline is somewhat of a trick question: of course the effect did exist before the railroads. However, it wasn’t yet defined by the physics of the day, nor was it even noticed before the ...

The Doppler effect is an observed change in pitch (how high or low a sound is) when either the source of the sound or the listener is in motion. Imagine a car, its horn blaring, approaching you as you ...

What if the speed of light is a constant only most of the time? What if gravity sometimes pushed instead of pulled? Scientists are increasingly asking what would seem like far-out questions regarding ...

Everyone has experienced the Doppler effect. An ambulance's siren has a higher pitch as it approaches you, and a lower pitch when moving away. Now, imagine that the pitch goes down as the ambulance ...

A system for measuring speed that is based on the Doppler effect. It is used in police radar systems as well as for measuring the velocity of hurricanes and tornadoes. See Doppler effect. THIS ...

There are many different sensors that can be used to detect motion in a given environment. Passive InfraRed (PIR) sensors are the most used today, as they work by detecting moving heat signatures.