BRAINCAKEThe Doppler Effect and Red Shift
When a source of waves moves relative to an observer, the observed wavelength and frequency change: the Doppler effect. If the source moves away, the waves are stretched out, so the observed wavelength increases and the frequency decreases. If it moves towards the observer, the wavelength decreases.
For light, an increase in wavelength shifts spectral lines towards the red end of the spectrum: red shift. A decrease is a blue shift.
Astronomers compare the absorption lines in a star's or galaxy's spectrum with the same lines measured in the laboratory. The pattern of lines is the same, but shifted. The red shift z is defined, for speeds much less than c, by
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Key terms in this lesson
- absorption lines
- Dark lines in a star's or galaxy's spectrum produced where atoms in its outer layers absorb light at specific wavelengths.
More in Space
- Life Cycle of a Star
- White Dwarfs, Neutron Stars and Black Holes
- Trigonometric Parallax
- Standard Candles
- Hubble's Law
- The Age of the Universe
- The Expanding Universe
- Dark Matter
All 18 lessons in Space · All Edexcel A-level Physics topics