The Doppler effect named after Austria

Austria

Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...

n physicist Christian Doppler

Christian Doppler

Christian Andreas Doppler was an Austrian mathematician and physicist.-Life and work:Christian Doppler was raised in Salzburg, Austria, the son of a stonemason. Doppler could not work in his father's business because of his generally weak physical condition...

 who proposed it in 1842 in Prague

Prague

Prague is the capital and largest city of the Czech Republic. Situated in the north-west of the country on the Vltava river, the city is home to about 1.3 million people, while its metropolitan area is estimated to have a population of over 2.3 million...

, is the change in frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 of a wave

Wave

In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

 for an observer

Observer

Observer may refer to person who is observing. More specialised meanings follow.-Computer science and information theory:*In information theory, any system which receives information from an object....

 moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren

Siren (noisemaker)

A siren is a loud noise making device. Most modern ones are civil defense or air raid sirens, tornado sirens, or the sirens on emergency service vehicles such as ambulances, police cars and fire trucks. There are two general types: pneumatic and electronic....

 or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.

The relative changes in frequency can be explained as follows.