Doppler EffectDoppler Effect

The Doppler Effect is the motion induced change in The Doppler Effect is the motion induced change in the observed frequency of a wave.the observed frequency of a wave.

The effect can only be observed/seen due to the The effect can only be observed/seen due to the relative motion between the observer and the relative motion between the observer and the wave source.wave source.

A net motion towards the source causes a blueshift A net motion towards the source causes a blueshift – a shift to higher frequencies - in the received – a shift to higher frequencies - in the received beam. A net motion away from the source causes beam. A net motion away from the source causes a redshift.a redshift.

The degree/ extent of the shift is directly The degree/ extent of the shift is directly proportional to the observer’s velocity relative to proportional to the observer’s velocity relative to the source.the source.

http://webphysics.davidson.edu/Applets/Applets.html

Doppler again ?Doppler again ?

SpectroscopySpectroscopy

Recall - atom/molecule emits radiation in the form of Recall - atom/molecule emits radiation in the form of photons.photons.

This radiation can be picked by a This radiation can be picked by a spectroscopespectroscope, a device , a device that disperses this radiation.that disperses this radiation.

Spectroscopes have Spectroscopes have diffraction gratings that act like diffraction gratings that act like prismsprisms. .

SpectroscopySpectroscopy

3 types of 3 types of spectra: spectra: 1. 1. continuous, continuous, 2. emission 2. emission and and 3. 3. absorption.absorption.

•Kirchhoff’s LawsP. 89

Spectroscopy and Sources.Spectroscopy and Sources.

2. Low density hot gas.

1. A luminous solid or liquid or a sufficiently dense gas.

3. A cool thin gas absorbs certain wavelengths from a continuous spectrum.

Actual SpectraActual Spectra

Spectra from:Spectra from:The Sun:

Numbers are in nanometers

Spectral InformationSpectral Information

Doppler effect Doppler effect and line and line broadening:broadening: when a familiar when a familiar pattern of lines pattern of lines appears, but the appears, but the lines are displaced lines are displaced from their usual from their usual locations.locations.

Thermal Broadening -Thermal Broadening ->>

Spectral InformationSpectral Information

Rotation Rotation Broadening.Broadening.

Spectral InformationSpectral Information

Wien’s law: wavelength of peak emission ~ Wien’s law: wavelength of peak emission ~ 1/Temperature.1/Temperature.

What this says is that the hotter the object the bluer What this says is that the hotter the object the bluer its radiation and vice versa.its radiation and vice versa.

Temperature is measured in Kelvins (K) where Temperature is measured in Kelvins (K) where kelvins = degrees Celsius + 273 (page 73).kelvins = degrees Celsius + 273 (page 73).

Temperature is a direct measure of the amount of Temperature is a direct measure of the amount of microscopic motion within an object: the hotter microscopic motion within an object: the hotter the object, that is the higher it’s temperature, the the object, that is the higher it’s temperature, the faster its constituent particles move and the more faster its constituent particles move and the more energy they radiate.energy they radiate.

Summary of spectral information.Summary of spectral information.Peak frequency or Peak frequency or wavelength wavelength (continuous (continuous spectra only)spectra only)

Temperature (Wien’s Temperature (Wien’s law)law)

LinesLines Composition, Composition, temperaturetemperature

Line widthLine width Temperature, Temperature, turbulence, rotation turbulence, rotation speed, density, speed, density, magnetic fieldmagnetic field

Doppler shiftDoppler shift Line-of-sight velocityLine-of-sight velocity