Wavelength shift for moving objects.
The wavelength of
light emitted by a moving object is shifted. This effect is called the doppler
shift.
- If the object is coming toward you, the light is shifted toward shorter
wavelengths, blue shifted.
- If the object is going away from you, the light is shifted toward longer
wavelengths, red shifted.
- The amount of shift is bigger if the emitting object is moving faster.
We don't normally notice this for light. But it is easy to observe for
sound:
- Blue shifted = higher frequency = higher pitch.
- Red shifted = lower frequency = lower pitch.
The
reason for this effect is easiest to understand from a picture. The object emits
one wave per second. At successive one second times it was located at the
positions of the blue dots ...
To get it really right, you should
understand the Special Theory of Relativity, but the picture gives the
qualitative idea.
Use of the Doppler shift in astronomy
- The amount of shift is bigger if the emitting object is moving faster.
- Thus if we know the original wavelength, we can deduce the speed.
- For a spectral line from an element that we can identify, we do
know the wavelength.
- Thus for many astronomical objects we can tell
- Is it coming toward us?
- Is it going away?
- If so, how fast is it moving toward us or away from us?
- But note that this method does not tell us how fast it is moving
sideways.