The Doppler Effect

1. The Doppler Effect in Astronomy
Chin-Vern Tan
Section 202

2. How Astronomers
Use the Doppler Effect
• Sounds from objects moving
toward you are at a higher
pitch
• This is because the sound
waves are compressed
together and the wavelengths
of the sound waves are
shortened
• Sounds from objects moving
away from you are at a lower
pitch, because the sound
waves are stretched apart and
the wavelengths are
lengthened

3. How Astronomers
Use the Doppler Effect
• Astronomers use the Doppler
Effect to study the motion of
the universe
• Ex. How nearby planets move
and how distant galaxies
expand
• Because of the Doppler Effect,
we know the universe is
accelerating apart

4. { the question }
A star emits visible colour at 550nm. If the star
is moving away from a stationary observer, can
a wavelength of 500nm be observed?

5. How to Approach
This Question
• Read the problem carefully and if
possible, do a rough sketch of the
problem
• Identify known and unknown
variables in the equation
• Plot a strategy to find the answer—
if there is more than 1 unknown
variable, is there another way to
obtain one of the unknown
variables with the information you
already know?
• Identify the equations you need to
use in order to solve the problem
and through algebraic manipulation,
solve for the unknown quantity

6. The Solution
• Identify what you know and what you need to find
• We know the wavelength is 550nm and that the source is
moving away from a stationary observer.
• Identify the equation to use

7. The Solution
• We do not need to actually do any calculations for this
example, if we fully understand the Doppler Effect!
• Using the first equation, we know that if the source is
moving away from the observer, the denominator will be
larger, as the speed of the source is negative
• Thus, the perceived frequency is lower than the actual
frequency of 550 nm

8. The Solution
• Thus, using the second equation, we can isolate for
wavelength
• Velocity remains constant, because it is simply the speed
of light
• Frequency will change, because the source is moving
away. From the Doppler Effect, we know that frequency
will be larger.
• Thus, the wavelength will be smaller than 550nm. It is
possible for a wavelength of 500nm to be observed.