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doppler effect

Light is characterized by its wavelength L



L = 4 10^-7 m is blue
L=5 10^-7 m is green
L = 7 10^-7 is red
larger is infrared, radio wave
shorter is ultra violet, xrays, gamma rays.

more on wavelengths and colors

The Doppler shift is a way of measuring the velocity of a moving source of light (or sound).
The observed wavelength changes depending on the  relative motion of source and observer.
Suppose the observer is stationary. if the source moves toward the observer,  the
wavelength L is shorter compared to the rest wavelength Lo , the shift L-Lo  is negative.
we call it blue shift because the light is pushed toward the blue end of the spectrum.

If the motion of the source is away from the observer, the wavelength L is longer.
The shift L-Lo is positive. the light is pushed toward the red end of the spectrum.
we call it a red shift.
Here 3 applets to understand:
applet 1       applet 2      applet 3

If we compute the shift, we can compute the " radio velocity" of the source relative to the
observer. To find the shift, we need to know Lo the rest wavelength.
If we know the type of star we are looking at or the type of supernovae
we know the color it is supposed to be so we can find the shift.

The equation that related the shift and the velocity of the source is :


If the shift is negative , the object is moving toward us.
For speed close to the speed of light, the formula needs to be modified, but still easy.

Example1: How fast you need to drive to make red light green so you can tell the policeman (if he took Astronomy 101)
you didn't know it was red ?
Lo = red , L= green  (observed) note that if the speed is close to the speed of light , the formula needs a modification
but this is fine for now)

do this activity to learn about the redshift of galaxies and hubble law
see this illustration