What is the rest mass of a photon traveling through water?

Let v be the speed of this photon, and c the speed of light in vacuum. Because v>0, the photon has some energy E>0. But the photon is traveling through water so v<c. From special relativity we know that E=m0*γ*c², with: m0=photon rest mass γ=1/sqrt(1-(v/c)²) Because v<c, γ>0. But E>0, then... show more Let v be the speed of this photon, and c the speed of light in vacuum.

Because v>0, the photon has some energy E>0. But the photon is traveling through water so v<c.

From special relativity we know that E=m0*γ*c², with:

m0=photon rest mass
γ=1/sqrt(1-(v/c)²)

Because v<c, γ>0.

But E>0, then m0>0.

But this is impossible because I read that photon rest mass is always 0, so where's my error?

Thank you.
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