Doppler Effect
Apparent change in the wave frequency (or in the period of periodic events)
due to motion
of wave source and/or observer is known as Doppler effect.
In the 17th century (well before
Doppler effect was predicted), Roemer
(Danish astronomer) estimated the speed of light from
the change in the
revolution period of one of Jupiter's moons. When the earth is approaching
Jupiter, the period is shortened, while receding, the period is elongated.
The animations show
wave patterns (surfaces of constant phase) emitted
by a statinary (top) and moving (middle)
wave sources. Wavelengths in front
of the moving source are squeezed (shortened) and those
behind are elongated.
Therefore, the frequency of a wave source approaching (receding from)
an
observer is higher (lower) than the "true" frequency. Similar effect occcurs
when the
observer is moving. For sound waves, the velocities of source
and observer are well defined
relative to air, the wave medium. For stationary
sound source and moving observer, the sound
speed appears to change (the
wavelength remains the same). For electromagnetic waves in vacuum,
the
velocity with respect to the wave medium (vacuum) is ill defined. In Doppler
effect of
electromagnetic waves, only the relative velocity between wave
source and observer matters.
Some objects (e.g., supersonic
planes) can move faster than sound.
Animation below shows wave fronts emitted by a supersonic
object
when the
Mach number is 2. No waves can exist outside the cone created by the object.
When
the cone surface hits an observer, sudden arrival of a sharp pulse
is detected as shock wave.
In vacuum, nothing can travel faster than light.
However, in materail media
(glass, plastic, etc.), electromagnetic waves
propagate in general at a speed slower than c.
Therefore, if a relativistic
charged particle with a velocity close to c enters, say, a plastic
body,
electromagnetic shock wave is emitted. This phenomenon is called Cerekov
radiation and is
in wide use for detecting energetic particles.
To repeat animation, click "Back" then "Forward"
buttons.
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