Which observation cannot be explained by treating light purely as a wave?
- A. Interference
- B. Diffraction
- C. Polarisation
- D. The existence of a threshold frequency in the photoelectric effect
Explanation
A wave of any frequency should eventually deliver enough energy if it shines long enough or brightly enough, so the sharp frequency threshold makes no sense on the wave picture and demands photons. Interference, diffraction and polarisation, by contrast, are precisely the phenomena that establish the wave nature. Light therefore needs both descriptions.
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