The photon perceives the entire journey as instantaneous. Individual photons always travel at C. The EM waves propagate slower in materials like glass and water, but the individual photons do not slow down. So the thickening atmosphere part of your story is technically a little off. The speed of light is slower but the photons are not. It has to do with waves interference patterns.
Skip to 5:15 in this video for a better explanation than I could ever give.
The photon perceives the entire journey as instantaneous. Individual photons always travel at C. The EM waves propagate slower in materials like glass and water, but the individual photons do not slow down. So the thickening atmosphere part of your story is technically a little off. The speed of light is slower but the photons are not. It has to do with waves interference patterns.
Skip to 5:15 in this video for a better explanation than I could ever give.
https://youtu.be/HZD4MR0KgqA?si=j1Q7PvXZHHXANewV