I was reading about light in a book there it was there that light is not a physical quantity, light has no mass as photons are more and more smaller then atoms. Then how does gravity affect the path of light? or it was written wrong.
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Gravity affects anything that carries energy. Photon has energy, therefore gravity attracts it (all men are mortal, therefore Socrates is mortal).
If it is the purported masslessness of photon that confuses you, then know that photon only has zero rest mass. If, on the other hand, you define mass as $m=E/c^2$ ("dynamic mass"), then it is obviously nonzero even for a photon and the paradox is no more.
If you are still not convinced, then refer to Einstin's GR equation and see that gravity couples to the stress-energy tensor, not mass.
John
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