The dual nature of light l wave theory

The dual nature of light l wave theory of light explains most phenomena involving light: n n n l wave theory does not explain: n n l propagation in straight line reflection refraction superposition, interference, diffraction polarization Doppler effect frequency dependence of thermal radiation photoelectric effect IS LIGHT A WAVE OR A PARTICLE? n n answer: it is both, depending on what question you ask: it has a “wave'’ aspect and a “particle” aspect Note: according to quantum theory, “particles” (e. g. electrons, protons, . . ) have also a “wave” aspect! (depends on what question you ask)

Thermal radiation n n l experimental observations: u atoms of a hot solid emit radiation; u increase in temperature more radiation, and component of maximum intensity shifted towards higher frequency (shorter wavelength) “classical” explanation: the hotter the solid, the more vibrational energy higher frequency of vibration of atoms/electrons higher frequency of radiation but frequency spectrum of this radiation (“black body radiation” calculated within framework of electromagnetism and thermodynamics did not agree with measured spectrum; predicted “ultraviolet catastrophe” I f 4 Max Planck's hypothesis (1900): energy is quantized; “oscillators” (oscillating atoms) can only have certain amounts of energy n n n relation between energy and frequency of oscillator: E = h f, where h = “Planck’s constant” = 6. 63 x 10 -34 Js calculation of black body spectrum using Planck's hypothesis gives formula (“Planck formula”) which describes measured spectra. = first evidence that energy is quantized

Photoelectric effect l l (first observed by Heinrich Hertz in 1887) electrons are emitted when certain metallic materials exposed to light (now used in photocells in cameras, and solar energy cells) some aspects of photoelectric effect could not be explained by classical theory: n classical theory: if light continuos flow of e. m. energy takes some (calculable) time for wave to supply sufficient energy for electron to be emitted; n find experimentally: current flows almost immediately upon exposure to light; n classical theory: light of any frequency could cause photoelectric effect - need only sufficient intensity n find experimentally: only light with frequency above certain minimum frequency causes electrons to be emitted n classical theory: energy of electrons depends on light intensity n find experimentally: energy of electrons depends on frequency Albert Einstein's explanation: n n n assume that not only energy in atoms is quantized, but also energy carried by light comes in “packets of energy” called light quanta or photons energy of one photon = h f, where f = frequency of the light. with this assumption, all aspects of photoelectric effect could be explained photon energy vs color of light: E = hf = hc/ blue light has more energy than red light