What physical phenomenon is shown in the picture?
Euler spiral 
Fraser spiral 
Mach number
Doppler effect
The Doppler effect  is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842. The Doppler effect is based on the principle of radar or ultrasound.
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Answers by country
Euler spiral 
Fraser spiral 
Mach number
Doppler effect
answers
 the Netherlands
the Netherlands17
 Australia
Australia19
 India
India65
 the United States
the United States57
 Poland
Poland1701
 Germany
Germany41
 the United Kingdom
the United Kingdom35
 Greece
Greece12
 Republic of Ireland
Republic of Ireland10