<p align="right">Last Update: <font color="#4f81bd">November 25, 2024</font></p> ## BIG IDEAS - A beam of light hits the surface of a metal and ejects an electron (photoelectron). - Observed by Heinrich Hertz in 1887. - When ultraviolet light shines on two metal electrodes with a voltage applied across them, the light changes the voltage at which sparking takes place. - The effect is measured by an <font color="#f79646">electroscope</font>. - Classical physics predicts any color of light (any frequency) will eject electrons if intensity is high enough. - Exists a threshold frequency; only light with a frequency greater than the threshold value causes electrons to be ejected. --- ### Definition A light of a certain wavelength striking a metal surface may cause the emission of electrons. This is called the **photoelectric effect**. The electrons emitted are called _photoelectrons_. The surface is known as _photosensitive_. ### Classical Physics [[Notes Vault/Physics Notes Vault/Physics Skills/Physics#Classical Physics|Classical physics]] cannot explain the photoelectric effect. All wavelengths of light have energy; therefore, classical physics would predict all wavelengths of light to cause emission of electrons when striking photosensitive material. ### Modern Physics [[Albert Einstein]] applied [[Max Planck|Planck's ideas]] and concluded there are packets of light later called [[Photon|photons]]. Instead, there is a minimum threshold frequency ($f_t$). This minimum frequency varies with the type of metal. **Guiding Question**: How do I describe the concept of the photon as an elementary particle and force carrier (quanta of energy)? - Photons are discrete bundles of light energy that obey Planck’s hypothesis of energy quantization. - Einstein’s photon model describes a beam of light as a beam of particles, each carrying energy . - The term photon was not used until 1926; the term was introduced by G.N. Lewis. **Guiding Question**: How do I explain how the solution to the photoelectric effect (photon) changed physics? - G.N. Lewis defined a photon as quantum energy. - The energy of a photon is determined solely by its frequency. **Guiding Question**: How do I state and explain that is the maximum kinetic energy of photoelectrons? - The minimum amount of energy necessary to eject an electron from a particular metal is referred to as the work function ( or ). **Guiding Question**: How do I solve problems involving kinetic energy, work function, threshold frequency, and stopping potential in the photoelectric effect, ? - Planck’s constant - Electronvolt () is a unit of energy. One is defined as the energy gained by one electron that has been accelerated across a potential difference of 1 v in a vacuum. Example What is the work function of a material whose threshold frequency ( ) is 4.2 x 10¹⁵ Hz? ### Example: A light shining on the metal phosphate will not emit electrons until the wavelength is less than 430 nm. >[!note] Recall that wavelength and frequency have an inverse relationship. As wavelength goes down then the frequency goes up. ### Related Topics --- [[Home|Home]] | [[Quantum mechanics]] | [[Notes Vault/Physics Notes Vault/Quantity of Motion/Quantum Mechanics/Blackbody Radiation|Blackbody Radiation]] | [[Photoelectric Effect]] | [[Atomic Theory]]