Question: Why do photons slow down as they exit the glass and what gives them the energy to accelerate?

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Soumendra Nath Thakur Answered:

The fact is that when a photon (hf) hits a glass surface, it is absorbed by an electron in an atom of the glass surface, and so the absorption of the incident photon energy moves the electron to a higher energy level, because such action destabilizes the electron, another photon (hf - hΔf) is emitted, in the same direction as the incident photon, released by the electron within the transparent glass, but the new photon carries slightly less energy (hf - hf') than the incident photon, as if the incident photon continued its journey with some absorption loss. This process is followed by similar absorptions and emissions until a new photon is released from the surface on the other side of the mirror. As a result some of the energy of the released photon is lost due to absorption losses (hf' = ∫ hΔf dn number of absorptions) in the transparent glass and accumulated delays relevant. The newly released photon maintains its intrinsic momentum with a lower energy than the incident photon, following a slightly deviated path.

When photons pass through the glass, some of their energy is absorbed and subsequently re-emitted due to interactions with electrons within the glass atoms. This absorption and re-emission process leads to a loss of energy for the photons, causing a decrease in their speed while they are within the glass medium. Unlike in free space, where the speed of light (c) is constant and can be represented by the equation c = f·λ (where c is the speed of light, f is the frequency, and λ is the wavelength), this equation does not fully apply within the glass medium due to the interactions with the electrons.

Upon exiting the glass into a vacuum, the photons do not regain the lost energy. Instead, they experience fewer interactions and absorption events, allowing them to continue their journey with the same reduced energy level. In free space, photons travel at the speed of light (c) and maintain a constant velocity. Therefore, they do not accelerate upon exiting the glass but continue to move at the same speed they had before entering the glass.