Wave-particle duality and matter waves
Quantum objects cannot be described completely as either classical particles or classical waves. Different experiments reveal complementary aspects of one quantum description.
1. Light as photons
Interference and diffraction show the wave aspect of light, while energy exchange occurs in localized quanta called photons. A photon of frequency f and wavelength λ has E = hf = hc/λ and momentum p = h/λ = E/c. Here h is Planck's constant and c is the speed of light. Shorter wavelength means higher frequency, energy and photon momentum.
2. Photoelectric evidence
When light strikes a suitable surface, electrons are emitted only if photon energy exceeds the work function φ. The maximum kinetic energy follows Kmax = hf − φ, and the threshold frequency is f0 = φ/h. Increasing intensity above threshold supplies more photons per second and can increase the number of emitted electrons, but it does not increase the energy of each photon. Increasing frequency increases photon energy and therefore Kmax.