15-Minute
Mini
LAB
Simulate the
Doppler Effect
Materials
• buzzer
• tape
• ball
Redshift and Blueshift
The motion of a star away from Earth causes the star’s spectral lines to shift toward
the red end of the spectrum. In the second spectrum below, the hydrogen lines have
moved left, indicating a redshift. When a star is moving toward Earth, its spectral lines
shift toward the blue end of the spectrum, as shown in the third spectrum below,
where the lines of the hydrogen emission spectrum have moved right, indicating a
blueshift.
Emission spectrum of hydrogen
Procedure
1
Tape the buzzer to the ball.
2
Have two people stand about
10 meters apart. Stand
between them.
Redshift
3 Turn the buzzer on, and have
them toss the ball back and
forth over your head.
Blueshift
Analysis
Describe what you hear. What
would you expect to hear if the
ball were thrown faster? slower?
Why can’t you do this experiment
with light instead of sound? If you
could somehow do this experiment
with light, what would you expect
to see?
Similarly, if a star is moving away from Earth, the wavelengths of light
increase, just as the sound waves of the siren did after the fire engine
passed you. All the star’s spectral lines shift toward the red end of the
spectrum, where wavelengths are longer. Astronomers call this
phenomenon a redshift.
From the blueshift or the redshift of a star’s spectrum, astronomers can
also determine how fast a star is moving toward or away from Earth.
Dividing a spectral line’s shift in wavelength by the wavelength determined
in the laboratory gives astronomers the ratio of the star’s velocity to the
speed of light. For example, if a star has a redshift of 0.001, it is moving
away from Earth at one-thousandth the speed of light, or about 300
kilometers per second.
28.1 Section Review
1
What do radio waves, visible light, and gamma rays all have in
common?
2
What does a spectroscope do?
3 How do scientists use different spectra to figure out the composition
of a star’s outer layer?
4
Why is the light reaching us from some celestial objects shifted
toward the red or blue end of the spectrum?
5 CRITICAL THINKING Every star has a unique spectrum. What does
this suggest about stars?
616
Unit 7 Space