Photoelectric Effect And Quantum Mechanics

The Ultraviolet Catastrophe Classical physics could not explain the distribution of radiation from a hot body. (Intensity as a function of wavelength)

Oscillators Max Plank postulated that energy was emitted by resonators which could only absorb and give off discrete amounts of energy. The energy of each resonator is given by E = nhf where n is an integer, f is the frequency, and h is PLANK’S CONSTANT.

Quanta Because the energy from the resonators comes in discrete units (multiples of Plank’s constant x frequency) it is said to be quantized. The quantum of light is called a PHOTON. A “resonator” (atom or molecule) radiates or absorbs energy when it changes quantum states. This energy corresponds to the difference between two energy levels.

The Photoelectric Effect A beam of light shining on a metal surface adds energy to the atoms’ electrons, sometimes enough to loosen them from their atoms and “eject” them.

Predictions of Classical Phyiscs Classical physical laws predicted Any wavelength of light, if shined long enough, should add enough energy to eject electrons The stronger the intensity of the light the more electrons should be ejected The kinetic energy of the ejected electrons depend should depend on the intensity of the light.

Experimental Evidence Whether electrons are ejected or not is not a function of intensity but of FREQUENCY of the incident light beam. The kinetic energy of the ejected electrons depends on the FREQUENCY of the incident light beam, not the intensity. Even at low intensities, electrons are ejected almost immediately.

Einstein Einstein won a Nobel Prize, not for his work in relativity, but for explaining the photoelectric effect. He proposed that electromagnetic waves are quantized. EM waves can be understood as a stream of particles (photons) Each photon has Energy = hf

Threshold Frequency The minimum frequency of light needed to eject an electron is called the threshold frequency. If the frequency of the incident light exceeds the threshold frequency, the photoelectric effect is observed; if the frequency is lower than the threshold frequency, no electrons are ejected.

Work Function The amount of energy each electron needs to escape the metal is known as the WORK FUNCTION. The work function is equal to hft where ft is the threshold frequency of the metal (and h is Plank’s constant). Different metals have different work functions.

Classical Predictions vs. Observations