When a photon enters a medium, such as a transparent material like glass or water, its speed decreases. However, it is important to note that even in a medium, the photon still does not have a reference frame of its own.
In the theory of relativity, the speed of light in a vacuum, denoted as "c," is a fundamental constant and the same for all observers regardless of their reference frame. It is always measured to be the same value, approximately 299,792,458 meters per second (or about 186,282 miles per second).
When a photon enters a medium, it interacts with the atoms or molecules of that material, leading to various phenomena such as absorption and re-emission. This interaction causes a delay in the photon's propagation through the medium, resulting in a reduction in its speed compared to its speed in a vacuum.
While the photon's speed changes in the medium, it is important to understand that the concept of a reference frame is tied to observers or objects with mass. Photons, being massless particles, do not have a rest frame or a reference frame of their own.
Regarding the hypothetical clock on the photon's back, it is not meaningful to discuss such a scenario because clocks require reference frames to measure time. Since the photon itself lacks a reference frame, the concept of a clock attached to it does not apply.
In summary, when a photon enters a medium and slows down, it does not acquire a reference frame, and the notion of a clock on its back is not applicable within the framework of our current understanding of physics.