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#### Will

**Doppler and photoelectric effect**

When using the doppler effect equations to determine the speed of celestial objects, what happens to the energy of the photons? If a certain device required a 600nm wavelenth of light and frequency 5E14 Hz to induce current, would photons from a celestial body moving away from Earth and emitting this wavelength have sufficient energy to induce current, since observed f is less than 5E14 Hz and E=hf ?

I was curious as to how astrophysicists can distinguish between one lightsource traveling at v1 with frequency f1 and another with v2,f2 if the observed f is the same for both.

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