Physics
Physics
Problem
During a classroom discussion a teacher states, “Light slows down in water because each photon loses energy as it enters the water.” Evaluate the validity of this statement. (a) Using appropriate physical principles, explain whether the energy of an individual photon changes when it passes from air into water. (b) Identify which physical properties of the photon do change upon entering the water and provide a reasoned explanation for those changes. (c) Describe one experimental observation, such as from the photoelectric effect or another modern-physics experiment, that supports your conclusions in parts (a) and (b).Intuitive explanation
This statement mixes a correct observation with a misconception about why it happens. Light does slow down in water—that's real—but photons don't lose energy; instead, their frequency stays constant while their wavelength shrinks. The key insight is that the speed of light in a medium depends on how the material's electrons interact with the electromagnetic wave, not on energy loss. When light re-emerges into air, it speeds back up to with the same frequency and energy intact, which wouldn't happen if photons had actually lost energy.
Intuitive explanation
This statement mixes a correct observation with a misconception about why it happens. Light does slow down in water—that's real—but photons don't lose energy; instead, their frequency stays constant while their wavelength shrinks. The key insight is that the speed of light in a medium depends on how the material's electrons interact with the electromagnetic wave, not on energy loss. When light re-emerges into air, it speeds back up to with the same frequency and energy intact, which wouldn't happen if photons had actually lost energy.