Explain how light interference or diffraction supports a wave description, and how the photoelectric effect supports a particulate description. Include a specific observation for each and explain why increasing intensity at a sub-threshold frequency does not reproduce the effect of increasing frequency.

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Interference/diffraction shows superposition and spreading; photoelectric threshold and K_max versus frequency support hf energy packets. Intensity increases photon rate, not energy per photon. Identify a concrete wave observation such as dark interference fringes from overlapping coherent light or a single-slit diffraction pattern. Explain it using phase, superposition or wave spreading rather than merely naming 'wave'. Identify the photoelectric frequency threshold or frequency-dependent maximum electron energy. Explain that one photon transfers hf; emission requires hf ≥ Φ. At fixed frequency greater intensity gives more photons, each with unchanged hf, so below-threshold photons still cannot eject electrons in the model.

Compare two kinds of light evidence

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