The Doppler Shift

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The Doppler Shift

• The wave appears shifted in frequency from the original, because of the speed of the object from which it was emitted or reflected.

• An increase in frequency shows an object is moving toward you.

• A decrease in frequency shows an object is moving away from you.

• Click here to see a Doppler shift video

image is courtesy of Windows to the Universe, http://www.windows.ucar.edu.

Astronomy and the Doppler Shift• Astronomers can measure

the Doppler Shift of a moving object.

• The radio wave has a known frequency for stationary objects.

• If target is moving away, waves in the signal will be stretched, and will have a lower frequency.

• If the target is moving closer, waves will be compressed and will have a higher frequency.

• For more information and a cool video click:

DYK???

• Police radar guns transmit a continuous wave. The radar gun compares the frequency of the wave reflected (the echo) from the moving vehicle to the frequency of the transmitted wave from the gun, and displays the vehicle's computed speed.

Doppler Formula

The Doppler shift formula for a moving source:

where λ1 is the unshifted wavelength

λ2 is the shifted wavelengthc is the speed of light and

v is the velocity of the object emitting the light, relative to the observer

c

v

1

12

1

Let’s Try An Example

• A spectral line normally at 4000 angstroms is shifted to 3999 angstroms because of the motion of the source. What is the velocity of the source, along the line of sight?

What Are We Given?

λ1 = 4000A

λ2 = 3999A

and c = 3 x 108 m/s

So

Solve for v:

= [(3999 – 4000)/4000]•3 x 108

= -7.5 x 104 m/s

Is the source coming towards us or away from us?

cv

1

12

Towards us!

The Doppler shift is called a

“Red shift” if v > 0 (moving away) since the wavelength is getting longer and the frequency shorter

“Blue Shift” if v < 0 (moving closer) since the wavelength is getting shorter and the frequency higher

(recall that blue light is higher in frequency than red)

• To try another example click here.

• If not proceed to the next slide.

Uses of the Doppler Shift

• Radio astronomers can find the motion of a source (stars, galaxies, gases, etc.) by observing whether the emission or absorption lines in its spectrum are shifted in wavelength relative to their wavelengths at rest

A typical passive radar system.The range and Doppler shift can determined

Doppler Shift in Action

• A telescope in both the US and Japan will have different velocities with respect to a galaxy they are observing since the signal received at each antenna is characterized by a Doppler shift that depends on the relative position and motion of the antenna and the signal source