Photon torpedo's....Well, a photon is a light particle. The term photon torpedo only refers to the fact that it emits light as it travels to the target.
I offer a possible explanation.
I personally don't think that the word "Photon" in photon torpedo has anything to do with light photons in any way.
Instead I offer that photon torpedo is like modern day spearfish torpedoes, or sidewinder missiles, neither of which, respectively, has anything to do with a type of fish or snake. Most likely the photon torpedo moniker refers to a family of torpedoes that Starfleet has deployed for close to a century as their standard torpedo.
The sidewinder missile is now over a half century old, and is in it (fourteenth?) generation. If Starfleet liked the photon torpedo, and were willing to periodically modify it, the various "photorp" variants could conceivably remain in service from Kirk's time through Janeway's.