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Doppler Effect Equations for Light
by Ron Kurtus (18 March 2008)
When the source of light or electromagnetic radiation is traveling at a high speed, the Doppler effect will cause a shift in the observed frequency and wavelength. Since the speed of light is much greater than the speed of the source, an approximate equation can be used to determine the shift of the radiation. The shift in wavelength is used in astronomy to tell when a distant galaxy or star is moving toward the Earth (blue-shift) or away (red-shift). Equations are available for determining the new frequency and wavelength, as well as the velocity of the source.
Questions you may have include:
- What are the equations for calculating frequency?
- How do you calculate the wavelength shift?
- What are the equations for velocity?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
Useful tools: Metric-English Conversion | Scientific Calculator.
Frequency equations
In the case of visible light or electromagnetic waves, the speed of light is much greater than the typical speed of the source. In such a case, the standard Doppler effect equation, as used in calculating the effect in sound, can be approximated.
Standard Doppler effect equation
The standard equation for the observed frequency of a waveform for a moving source is:
fo = fv/(v ± vs)
where
- fo is the observed frequency
- v is the velocity of the waveform
- vs is the velocity of the source
- f is the emitted frequency
- ± is plus or minus; plus (+) is used when motion is away from you and minus (−) is used when motion is toward you
(See Doppler Effect Equations for Sound for more information on that application.)
Blue-shift equation
The equation for the observed frequency of a light wave when the source is traveling toward you (blue-shift) is:
fb = f(1 + vb/c)
where
- fb is the observed blue-shift frequency
- vb is the velocity toward you
- c is the speed of light
- c >> vb (c is much greater than vs)
Red-shift equation
The equation for the observed frequency of a light wave when the source is traveling away from you (red-shift) is:
fr = f(1 − vr/c)
where
- fr is the observed red-shift frequency
- vr is the velocity away from you
Note that "−" and "+" are reversed as compared to the standard Doppler effect equations. This is a result of the approximation.
Wavelength
A shift in frequency of electromagnetic radiation is not readily measured. Instead, devices such as a spectroscope is used to measure a change in wavelength of the light. Knowing the velocity of the moving source of light (vs), you can use the equations c = fλ and f = c/λ to convert the frequency equations to solve for wavelength.
The blue-shift equation for wavelength is:
λb = λc/(c + vb)
where
- λb is the observed blue-shift wavelength
- λ is the emitted wavelength (Greek symbol lambda)
The red-shift equation for wavelength is:
λr = λc/(c − vr)
Example
When heated, the various chemical elements give off light in a specific series of wavelengths or spectral lines. Astronomers study a number of spectral lines, but for our purposes, we will use the Hydrogen spectral line of λ = 434 nm, which is in the violet region of the visible spectrum.
(See Electromagnetic Spectrum for more information.)
If a distant galaxy is moving away from us at approximately 50,000 km/s and we approximate the speed of light as c = 300,000 km/s, then the resulting wavelength will be:
λr = λc/(c − vr)
Substitute values into equation.
λr = 434*300000/(300000 − 50000)
λr = 520.8 nm
The line has shifted from violet to green. Other spectral lines would also have shifted toward the red end of the spectrum.
Velocity
Typically, an astronomer would study the spectrum of a distant star or galaxy and measure the new observed spectral lines of the various elements on the object. Then the direction and velocity of the star or galaxy would be determined.
The equation for velocity is derived from the above wavelength equations.
The blue-shift equation for velocity is:
vb = c(λ/λb − 1)
The red-shift equation for velocity is:
vr = c(1 − λ/λr)
Example
If the astronomer saw that the Hydrogen 434 spectral line of a distant star had shifted to 482 nm, what would be the velocity of the star?
Since the wavelength increased, you would use the red-shift equation.
vr = c(1 − λ/λr)
Substitute the values.
vr = 300000*(1 − 434/482)
vr = 300000*(1 − 0.9)
vr = 30,000 km/s
Summary
When the source of light or electromagnetic radiation is traveling, the Doppler effect will cause a shift in the observed frequency and wavelength and an approximate equation can be used to determine that shift. The shift in wavelength is used in astronomy to tell when a distant galaxy or star is moving toward the Earth (blue-shift) or away (red-shift). Equations are available for determining the new frequency and wavelength, as well as the velocity of the source.
Think of some clever new ways of doing things
Resources and references
The following resources provide information on this subject:
Websites
Books
Top-rated
books on Physical Science
Mini-quiz to check your understanding
If you got all three correct, you are on your way to becoming a Champion in Physics. If you had problems, you had better look over the material again.
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Doppler Effect Equations for Light