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Steve Jones
Studied at Imperial College LondonAuthor has 12.3K answers and 24.1M answer views5y
The kinetic energy of a photon is relative, not absolute. That’s because the kinetic energy of a photon is inversely proportional to its wavelength. Whilst the velocity of a photon is not affected by the relative velocity of the source and observer, the wavelength is. If the source of light is moving away from the observer sees it as “red shifted” with a longer wavelength, and therefore with lower energy, whilst the opposite happens if the source is moving towards observer.
This is due to something called the Relativistic Doppler Shift. Whilst the effect is nominally similar to the “everyday” Doppler Shift we observe with sound from moving vehicles, it’s due to a somewhat different relativistic effect.
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