PHYSICS 20B Lecture Notes - Lecture 8: Blueshift, University Of Manchester
Lecture 8
What we can learn via light
● Doppler Effect
○ Example with ambulance
■ When an ambulance is moving away from a person, the person hears a
lower pitch, because of the longer wavelength (redshift)
■ When an ambulance is moving towards a person, the person hears a
higher pitch, because of the shorter wavelength (blueshift)
○ By measuring the difference of the shifts (blueshift/redshift), we can determine
the object’s velocity toward or away from us
○ Note: if the object is not moving relative to the observer, then the observed
spectrum will not be Doppler-shifted
■ Like if you and your friend are on a merry go round together, your friend is
moving, but not relative to you
● Doppler shift of light
○ If the object moves towards or away, the observer sees the respected spectrum
(blueshift/redshift)
○ If the object is moving upwards, or downwards, in respect to the observer, the
observer receives light that is not Doppler-shifted
● Calculating speed of object’s movements
○ If we can find the wavelength at rest, and wavelength when moving away, we
can find the speed
■ The change of wavelength, divided by the wavelength at rest equals the
velocity of the object divided by 3 x 10^8 m/sec (C, constant)
■ So if we multiply the constant to the division of the changed wavelength
and wavelength at rest, it will give us the velocity
● What light can tell us (Important)
○ Color of light emitted by an object = object’s temperature
■ Hotter objects = blue, colder objects = red
● Hotter objects = shorter wavelengths, Colder objects = longer
wavelengths
○ Doppler Shift = Velocity of tan emitting object
○ Different atoms/molecules emit/absorb light at specific wavelengths =
composition of objects
○ The fainter, the further = distance to a given object
● Apparent Brightness
○ A star’s apparent brightness will depend on intrinsic luminosity(power) and
distance to the star
○ The apparent brightness equals the luminosity of the object divided by 4 times pi
times distance to the star(squared)
■ 4 times pi times distance to the star(squared) is the surface area of a
sphere
● The Past
○ Light (in a vacuum) travels at a fixed and finite speed:
find more resources at oneclass.com
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Document Summary
When an ambulance is moving away from a person, the person hears a lower pitch, because of the longer wavelength (redshift) When an ambulance is moving towards a person, the person hears a higher pitch, because of the shorter wavelength (blueshift) By measuring the difference of the shifts (blueshift/redshift), we can determine the object"s velocity toward or away from us. Note: if the object is not moving relative to the observer, then the observed spectrum will not be doppler-shifted. Like if you and your friend are on a merry go round together, your friend is moving, but not relative to you. If the object moves towards or away, the observer sees the respected spectrum (blueshift/redshift) If the object is moving upwards, or downwards, in respect to the observer, the observer receives light that is not doppler-shifted. If we can find the wavelength at rest, and wavelength when moving away, we can find the speed.
