PHOTOELECTRIC EFFECT Z.H.MANE

Photoelectric Effect Metal Foil

Photoelectric Effect Metal Foil

Photoelectric Effect As blue light strikes the metal foil, the foil emits electrons.

Photoelectric Effect

When red light hits the metal foil, the foil does not emit electrons. Blue light has more energy than red light. Try increasing the brightness.

Photoelectric Effect

Well, that didn’t work! Maybe its still not bright enough.

Photoelectric Effect

Still not working. What happens with brighter blue light?

Photoelectric Effect

More blue light means more electrons emitted, but that doesn’t work with red.

Photoelectric Effect

Source: Photoelectric Experiment

CHARACTERISTICS OF PHOTOELECTRIC EFFECT Threshold Frequency ν 0 Intensity & Photocurrent Maximum K.E. & Frequency Instantanious effect

Photoelectric Effect quanta. ν Einstein said that light travels in tiny packets called quanta. The energy of each quanta is given by its frequency ν E=h ν Energy Planck’s constant frequency

Photoelectric Effect Each metal has a minimum energy needed for an electron to be emitted. work function This is known as the work function  0 h So, for an electron to be emitted, the energy of the photon, hν, must be greater than the work function,  0 kinetic energy The excess energy is the kinetic energy of the emitted electron.

EINSTEIN’S PHOTOELECTRIC EQUATION h ν =  + ½ mv 2

Photoelectric Effect (Quantum Theory of Light) Max Planks created the quantum theory of light, which states that electromagnetic radiation traveled as tiny packets of energy called quanta, or photons, that behaved like particles called photons, rather than continuous waves. E instein's work on quanta of light provides the foundation for Photoelectric Effect

Albert Einstein received the Nobel prize in physics in 1921 for explaining the photoelectric effect and for his contributions to theoretical physics.

What is the Photoelectric Effect? A photon with energy (h ν ) strikes an electron and ejects it from the piece of metal. The energy does work to remove electron from metal W 0. The remaining energy is converted into kinetic energy (KE) of the ejected electrons.

It's been determined experimentally that when light (a photon of energy) strikes a metal plate, photoelectrons will be ejected from the metal. The energy does work to remove electrons from metal. The remaining energy is converted into kinetic energy of the ejected photoelectrons. The Photoelectric Effect

No matter how many photons strike the metal, if none of them has sufficient energy to eject an electron from a metal atom, you won't get a current. If the energy the taken up by the electron is sufficient to allow it to be released from the metal atom, you will get a current. Here the photon is absorbed by the electron and ceases to be a particle.

Photoelectric effect

Shine light rays on metal --> produces current Increase intensity of light rays (total energy) --> current goes up The strange thing about this phenomenon, is that a certain minimum frequency of light is required before any current is detected called threshold frequency The higher the frequency of light - the higher the energy of the photon

Higher intensity light has more photons, and so will knock out more electrons. However, if the frequency of the light is such that a single photon is not energetic enough to release an electron from the surface, then none will be ejected no matter how intense the light. One would expect the energy of the emitted electrons to depend on the intensity of the light -- but it does not.

In Einstein's model, increasing the intensity of the light would cause greater numbers of electrons to be ejected, but each electron would carry the same average energy. I the frequency, rather than the intensity, of the incident radiation would increase the average energy of the emitted electrons.

Einstein’s Theory The photoelectric effect is interpreted with photons and the conservation of energy with the equation: h ν =  + ½ mv 2 h ν equals the energy of each photon Source:

Kinetic energy of emitted electron vs. Light frequency Source: content/PhyAPB/lessonnotes/dualnature/ photoelectric.asp

Source: Photoelectric Experiment

Use this schematic to construct the experiment. + - Flow of current 9 volts Quick Check: Your circuit should like this one. But you should be able to build it looking at the schematic alone! Photocell Copy and label all the components of the schematic in your notes. The arrows are part of the schematic symbols.

Applications Solar panels are nothing more than a series of metallic plates that face the Sun and exploit the photoelectric effect. The light from the Sun will liberate electrons, which can be used to heat your home, run your lights, or, in sufficient enough quantities, power everything in your home. Source: picsolarpannelsmatt.jpg

Photoelectric Smoke Detector Source:

Photoelectric Effect Applications Photoelectric Detectors In one type of photoelectric device, smoke can block a light beam. In this case, the reduction in light reaching a photocell sets off the alarm. In the most common type of photoelectric unit, however, light is scattered by smoke particles onto a photocell, initiating an alarm. In this type of detector there is a T-shaped chamber with a light-emitting diode (LED) that shoots a beam of light across the horizontal bar of the T. A photocell, positioned at the bottom of the vertical base of the T, generates a current when it is exposed to light. Under smoke-free conditions, the light beam crosses the top of the T in an uninterrupted straight line, not striking the photocell positioned at a right angle below the beam. When smoke is present, the light is scattered by smoke particles, and some of the light is directed down the vertical part of the T to strike the photocell. When sufficient light hits the cell, the current triggers the alarm. Source:

Applications OF PHOTO CELL BURGLAR ALARM LUX METER CINE FILMS EXPOSURE METER

Applications: Night Vision Device