3.12 Photoelectric Effect

The Photoelectric Effect Facts:

1: Highly intense low frequency light does not eject any electrons, even if it shines on the surface for several days 2: When the threshold frequency is reached, electrons are ejected immediately 3: Increasing the intensity of the light at a frequency that will cause electrons to eject results in a higher ejection rate. However, all ejected electrons share the same velocity. 4: Increasing the frequency of the light increases the velocity of the ejected electrons. However, all ejected electrons share the same velocity.

Planck's Constant

6.626 x 10^-34

Planck's equation

E = hv Energy per Quantum (J) = Planck's Constant x Frequency (Hz)

Wavelength

Horizontal distance between the crests or between the troughs of two adjacent waves

Einstein's Theory (1905)

A beam of light is a stream of particles called photons. The energy of a photon is related to its frequency according to E=hv. The quantum of Planck is a particle - a photon. If the frequency of the photon is below a certain threshold, no electrons are ejected. If the frequency of the photon is at or above a certain threshold, its energy is transferred to the electron. This causes the electron to overcome the forces of attraction holding it to the metal. The electron absorbs the photon.

Why do we have different colors of light?

As wavelength/frequency changes, color changes. But there is duality to light: it behaves like a wave and like a particle (a photon). As wavelength/frequency changes, the energy per photon changes. E=hv

Quantum Theory and Planck

Max Planck hypothesized that the energy radiated from a heated object is emitted in discrete units or quanta. We assume that energy increases in a continuous stream, but it actually increases in units. It increases by a full quantum, or not at all.

The Photoelectric effect

The emission of electrons from a metal when light shines on the metal

Absorbing Photons

When a photon is absorbed by an atom or molecule, an electron moves up one or more energy levels. The increase in energy is equal to the energy of the photon that was absorbed. The increase n energy is also equal to the difference in energy between the two energy levels.

Emitting Photons

When a photon is emitted from an atom or molecule, an electron moves down one or more energy levels. same exact thing as absorbing photos but decreasing instead b

The Formula

c=λv speed of light (3 x 10^8 m/s) = wavelength (m) x frequency (s^-1 or Hz)

Frequency

the number of times a wave repeats itself per second Red: low frequency, high wavelength Violet: high frequency, low wavelength