I am trying to wrap my head around the doppler effect in regard to light waves. I'm in an astronomy class and apparently in astronomy when a light source is moving away it appears redder (redshift). When the light source is moving toward you it will appear bluer (blueshift). But I'm not sure WHY this is. Can someone help explain this to me?
-
If an object is constantly emitting light waves, it's sending out pulses of light with a certain frequency that are all an equal distance from each other. But when this object is moving away from you, the pulses of light will be spread out. Once it sends out one pulse in your direction, the object moves a little distance away from you and sends out the next pulse. The distance between these pulses is greater than if the object was standing still.
What makes something look a certain color is its frequency. And when the wavelength of light is increased, frequency must go down because light always travels at the speed of light. Red is the lowest frequency light, and that's why things get red shifted.
The same logic applies to blue shifts, too, except the wavelength will be decreased from the object moving towards you, causing the frequency of the light to increase. Blue is one of the higher frequency colors of light.
What makes something look a certain color is its frequency. And when the wavelength of light is increased, frequency must go down because light always travels at the speed of light. Red is the lowest frequency light, and that's why things get red shifted.
The same logic applies to blue shifts, too, except the wavelength will be decreased from the object moving towards you, causing the frequency of the light to increase. Blue is one of the higher frequency colors of light.
-
Another way to look at Red-shift and Blue-shift is from Conservation of Energy.
We know from Special Relativity that the speed of light (photons) is a constant. Now think about:
You are standing on the ground next to a train track. A train going 100 km/hr is going past. On a flat car is a baseball pitcher that you know can throw 195 km/hr fastballs throwing toward toward you. You have a radar gun so you can measure the speed of the baseballs.
What speed would you measure if the train was coming toward you? (100 + 195 = 295 km/hr )
What speed would you measure if the train was going away from you? (195 - 100 = 95km/hr)
But, light does NOT work like the baseballs above. And energy must be conserved. Something has to give to account for the energy loss from the light of a star when the star is moving away from us. It cannot be speed, but something has to give. What gives is frequency of the photon. The frequency must decrease. The photons get redder. Turns out that due to Wave-Particle Duality the Doppler Effect Equations work for for light also.
Same thing for stars moving toward us. They are gaining energy from the motion of the star. The frequency must increase. The photons get bluer.
We know from Special Relativity that the speed of light (photons) is a constant. Now think about:
You are standing on the ground next to a train track. A train going 100 km/hr is going past. On a flat car is a baseball pitcher that you know can throw 195 km/hr fastballs throwing toward toward you. You have a radar gun so you can measure the speed of the baseballs.
What speed would you measure if the train was coming toward you? (100 + 195 = 295 km/hr )
What speed would you measure if the train was going away from you? (195 - 100 = 95km/hr)
But, light does NOT work like the baseballs above. And energy must be conserved. Something has to give to account for the energy loss from the light of a star when the star is moving away from us. It cannot be speed, but something has to give. What gives is frequency of the photon. The frequency must decrease. The photons get redder. Turns out that due to Wave-Particle Duality the Doppler Effect Equations work for for light also.
Same thing for stars moving toward us. They are gaining energy from the motion of the star. The frequency must increase. The photons get bluer.