The Doppler Effect in Astronomy

Chin-Vern TanSection 202

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

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

{ 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?

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

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

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

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.