3.12 Doppler Effect

Pre-Lecture Reading 3.12

Video Lecture

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Doppler Effect (12:40)

Supplementary Notes

Doppler Effect

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See Doppler Effect.
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Consider an emitter of waves. These waves get compressed in the direction of motion and decompressed opposite the direction of motion.

The amount of compression is given by:

( 15 )

Δλ

λ_em

v_wave

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Δλ = change in wavelength
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λ_em = emitted wavelength
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v = speed of emitter toward or away from observer
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v_wave = speed of wave

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For light, v_wave = c. Solving for Δλ yields:

( 16 )

Δλ =

× λ_em

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If the source is moving toward you (or you are moving toward it), the observed wavelength is shorter than the emitted wavelength, and hence the light is blueshifted:

( 17 )

λ_obs = λ_em − Δλ

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If the source is moving away from you (or you are moving away from it), the observed wavelength is longer than the emitted wavelength, and hence the light is redshifted:

( 18 )

λ_obs = λ_em + Δλ

Example:
A star is moving toward us at 1/1000^th of the speed of light. You take a spectrum of this star and identify a Balmer absorption line series. Balmer alpha (Hα) is always emitted at 656.5 nm. You observe it to be shifted by

Δλ

× λ_em

0.001 × 656.5 nm = 0.06565 nm.

Since the star is moving toward us, you observe it at

λ_obs	=	λ_em − Δλ
	=	656.5 nm − 0.06565 nm = 655.8 nm.

Exercise

Experiment with UNL's Doppler Shift Demonstrator.

Assignment 3

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Do Question 9.