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Doppler Effect
Mr. Bean
Discovery Science
The Doppler Effect
• Heard an ambulance go by recently? Remember how the siren's
pitch changed as the vehicle raced towards, then away from you?
First the pitch became higher, then lower. Originally discovered by
the Austrian mathematician and physicist, Christian Doppler (180353), this change in pitch results from a shift in the frequency of the
sound waves
• As the ambulance approaches, the sound waves from its siren are
compressed towards the observer. The intervals between waves
diminish, which translates into an increase in frequency or pitch.
• As the ambulance recedes, the sound waves are stretched relative
to the observer, causing the siren's pitch to decrease. By the change
in pitch of the siren, you can determine if the ambulance is coming
nearer or speeding away. If you could measure the rate of change of
pitch, you could also estimate the ambulance's speed.
The Doppler Effect
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By analogy, the electromagnetic radiation emitted by a moving object also
exhibits the Doppler effect.
The radiation emitted by an object moving toward an observer is squeezed;
its frequency appears to increase and is therefore said to be blue-shifted.
In contrast, the radiation emitted by an object moving away is stretched or
red-shifted.
As in the ambulance analogy, blue-shifts and red-shifts exhibited by stars,
galaxies and gas clouds also indicate their motions with respect to the
observer.
The Doppler Effect In Astronomy
• In astronomy, the Doppler effect was originally studied in the visible
part of the electromagnetic spectrum.
• Today, the Doppler shift, as it is also known, applies to
electromagnetic waves in all portions of the spectrum. Also, because
of the inverse relationship between frequency and wavelength, we
can describe the Doppler shift in terms of wavelength.
• Radiation is red-shifted when its wavelength increases, and is blueshifted when its wavelength decreases.
• A red-shift occurs when the source and observer are moving away
from each other.
• A blue-shift occurs when the source and observer are moving
towards each other.
Doppler Effect
• Blue shifted = higher frequency = higher pitch.
• Red shifted = lower frequency = lower pitch.
Here is what the spectrum of a star that is moving toward
us might look like (Blue-shift)
Here is what the spectrum of a star that is moving away
from us might look like (red-shifted)
Gravitational Red-shift
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According to General Relativity, the wavelength of light (or any other form of
electromagnetic radiation) passing through a gravitational field will be
shifted towards redder regions of the spectrum.
To understand this gravitational red-shift, think of a baseball hit high into
the air, slowing as it climbs.
Einstein's theory says that as a photon fights its way out of a gravitational
field, it loses energy and its color reddens.
Gravitational red-shifts have been observed in diverse settings.