Photon

aPhotonPhoton, also known as light quantum, is a minute energy packet of electromagnetic radiation. Einstein deduced that the light quantum could also be associated with energy which is directly proportional to frequency of the radiationE ∝ fE = hfWhere his Plank Constant whose value is 6.62 × 10−34 joule second.These photons are massless. But as they have energy E, According to Einstein Mass energy relation, E = mc2or, mc2=hfor, m = hf

c2and momentum = hf

c A substantial energy value and momentum strongly indicated that the light quantum could be associated with a particle. This particle was later named photon. Therefore, a photon is defined as a discrete bundle (or quantum) of electromagnetic (or light) energy. Since photons are massless, they also move at the speed of light (3 × 108 m/s). Basic Properties of PhotonAccording to the photon theory of light, we can summarize the properties of photons as• Each photon has momentum p (= h ν/c), energy, E (=hν) and speed c, the speed of light.• Photons are electrically neutral, and are not deflected by electric and magnetic fields.• Photons can be destroyed or created when radiation is absorbed or emitted, respectively.• The total momentum and total energy are conserved in a photon-particle collision.• All photons of light of a particular frequency and wavelength have the same energy.• Photons have zero resting mass.Difference between Photon and Electron

PHOTOELECTRIC EFFECTWhenever light or electromagnetic radiations such as x rays , ultraviolet rays fall on a metal surface. Some electrons are emitted from the surface. this phenomenon of emission from its metallic surface when radiations of suitable frequency falls on it is called photoelectric effect. These electrons are called photoelectrons.Metals like zinc, cadmium are more sensitive only to ultraviolet light where as alkali metals like sodium, potassium, are sensitive even to visible light.Experiment on Photolelectric Effect

For the investigation of the photoelectric effect a schematic diagram of the apparatus as used by Lenord (1902) is shown in the fig. Monochromatic light from the lamp illuminates a cathode plate P in an evacuated glass enclosure. A battery maintains a potential difference between P and a anode A, which collects the photoelectrons. The potential V can be varied to be either positive or negative relative to P. When the collector is positive with respect to the plate, the electrons are attracted to it and the ammeter (A) registers a current. Lenard studied the dependence of photoelectric current on the following factors. (i) Intensity of incident radiation (ii) Potential difference between the plate and the collector (iii) Frequency of the incident radiation The result of observations are as follows:Effect of the intensity of Incident Radiation

When the collector is positive relative to the plate and the potential difference is kept fixed, then for a given frequency of radiation, the photoelectric current is proportional to the intensity of the light, as shown in fig. It shows that the number of emitted photoelectrons is proportional to the light intensity. Furthermore, there is no threshold intensity. Effect of Potential Difference

When the frequency and intensity of radiation are kept constant and the positive potential of collector relative to plate is gradually increased, then the photoelectric current i increases with the potential difference V. At some value of the potential difference, when all the emitted electrons are collected, thus increasing potential difference has no effect on the current. The current has reached its maximum value, called the saturation current.When the polarity of the battery is reversed, the electrons are repelled and only the most energetic ones reach the collector, so the current falls. When the retarding potential difference reaches a critical value, the current drops to zero. At this stopping potential Vo, only those electrons with the maximum kinetic energy are able to reach the collector. 1

2mv2max= eVo1

2mv2max∝VoThus, the maximum kinetic energy of a phoelectron can be determined by knowing the value of the stopping potential. Variation of stopping potential with applied voltage is shown in figure above.For a given frequency of light, the saturation current depends on the intensity of light. Larger the intensity; higher the saturation current. However, the stopping potential does not change with the intensity. Effect of frequency

For a given intensity of radiation, the stopping potential depends on the frequency. Higher the frequency, higher the value of stopping potential. Threshold frequency

The maximum kinetic energy of the electrons depends on the light source and the plate material, but not on the intensity of the source. Certain combinations of light sources and plate materials exhibit no photoelectric effect. For a metal plate there exists a minimum frequency called threshold frequency (fo) below which no electron is emitted however large the light intensity may be. The threshold frequency is a characteristic of the metal plate.Work FunctionThe minimum energy of photon required just to liberate an electron from the metal surface with zero velocity is known as work function 𝜙 of that metal. It depends upon the nature of the metal , its purity and condition of the surface. laws of photoelectric emission• The photoelectric effect depends upon the frequency of the radiation it doesn't depend upon the frequency of the radiations.• the rate of emission is directly proportional to the intensity of the light .• the velocity and energy of photoelectrons depends upon the frequency of the radiations but does not depends upon the intensity.• the stopping potentials also independent to the intensity of radiations but it directly depends to the frequency of the radiations.• there is an instantaneous emission of photoelectric within the limits of experienced accuracy.Einstein's Photoelectric Equation

Einstein Proposed that• Radiation with frequency f consists of a stream of discrete quanta or photons, with energy hν, where h is Planck’s constant. Photons travel at the speed of light through space.• When photons and electrons in the emitter’s atoms collide when radiation of frequency f is incident on a photosensitive surface. During such a collision, the photon’s whole energy is transmitted to the electron with no time lag.An elecctron absorbs energy of photon hf and is utilized in two ways. The electron uses some of its energy to break free from the atom. The minimum energy required to free-electron from a given surface is called photoelectric work function 𝜙 of the material of the surface.The residual energy (h f- 𝜙) emerges as electron kinetic energy. If the electron does not lose any of its energy in impact with the surface and exits with the greatest possible kinetic energy. Energy of photon = Work function + K.E of electron hf= 𝜙 +1

2 m v2max …..(1) where,m = mass of the electronvmax = maximum velocity of electronAll the photoelectrons emitted from the metal surfaces do not have the same energy. If the photon is just able to eject electron from the atom and electron do not accelerate, then corresponding frequency of the photon is called threshold frequency f0. then eqn (1) becomes hf0= 𝜙 +0 𝜙=hf0 Now hf= hf0 +1

2 m v2max 1

2 m v2max= h(f-f0)This equation is Einstein`s photoelectric equation. Millikan's Experiment to determine Plank's Constant

The apparatus consists of an evacuated chamber (vaccum) where different alkali metals are placed on a rotating table so that experiment can be performed for different surfaces like Sodium, Potassium, Lithium etc. without evacuating the chamber. The metals are very reactive so they may form a film over the surface. so a scraper (knife) is inserted to surface. A sensitive galvanometer G and a voltmeter V are also connected to detect the current and measure the Pd across the metallic surface and the collector (which collects the electron).The Pd across the arrangement can be varied by rheostat. A light radiation o suitable frequency f (f > f0) is made incident on the metallic surface are collected at the other surface which faces the metallic surface. The galvanometer shows deflection due to photoelectric current. We know from Einstein’s photoelectric equation: hf=ϕ+1

2mv2max………(i) The potential difference across the arrangement is slowly increased until the galvanometer shows zero deflection. This corresponding potential difference is called stopping potential (V0). Then kinetic energy becomes equal to the stopping potential. So, 1

2mv2max=eV0……..(ii)From equation (i) and (ii), hf=ϕ+eV0f=ϕ+eV0 or,V0=hf0+(−ϕe)……..(iii)Here, Φ and hf0 is constant for a given metal. So, the euqaion (iii) represent the equation of straight line y = mx +c. Now, the frequency of incident radiation is changed and the corresponding value of stopping potential is measured. If we plot a graph between incident frequency (f) and stopping potential (Vs) we obtain a straight line having negative intercept whose slope gives the measure of h/e and intercept gives the measure of Φ/e.tan 𝜃 = h

eh = e tan𝜃Hence, measuring the value of tan𝜃 from the graph obtained and multiplying with charge of electron 'e', we can obtain the value of Plank's constant (h). Uses of Photolectric effectPrinciple of photoelectric effect is used in photolectric cells. There are mainly three types of photoelectric cells: 1. Photo- emissive cells 2. Photo- voltaic cells 3. Photo - conductive cells.1. Photo- emissive cells

A photoemissive cell, commonly known as a phototube, makes use of the photoelectric effect, the phenomenon whereby light-sensitive surfaces give off electrons when struck by light. Photoemissive cells are sometimes called photocells or electric eyes. A phototube consists of a vacuum tube, housed in glass or quartz, containing two electrodes: a curved surface called a cathode 'C' and a slender rod called an anode 'A'. The cathode is made of a photoemissive material with lower work fuction, so that when light strikes the cathode, it emits electrons and is very sensitive to the light. Therefore it is used in the photometry, television, burglar alarm etc . 2. Photo- voltaic cell

A solar cell or photovoltaic cell (PV cell) is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect. When light falls on a PN junction, a voltage difference builds up as a consequence of the absorption of photon energy. The generation of parallel voltage difference due to the effect of light on the PN junction is called Photo Voltaic Effect. The semiconductor which receives the light emits electrons and electrons are passed through another layer through a junction. Thus first layer forms holes ( +ve charge) and second layer is negatively charged due to excessive number of electrons. Thereby creating a potential diffference. 3. Photo-Conductive Cells ( light dependent Reisitors LDR)Photoconductive cells are light-sensitive resistors in which resistance is inversely proportionated to light intensity when illuminated. These devices consist of a thin single-crystal or polycrystalline film of composite semiconductor materials. Cadmium sulfide (CdS) is used to make the most commercially available photoconductive particles which are sensitive to light in the visible spectrum. They are used in light-sensitive alarms, automatic street lights, and lighting control system.