PHYSICS.pdf
- 1. CONTENT
- 2. PHOTOELECTRIC EFFECT The phenomenon of emission of electrons from mainly metal surfaces exposed to light energy (X – rays, γ – rays, UV rays, Visible light and even Infra Red rays) of suitable frequency is known as photoelectric effect. The electrons emitted in this process known as photoelectrons and the No index entries found.current constituted by this electrons known as photoelectric current. This phenomenon also known as photoemission. Photo – electric effect was first experimentally verified by H. R Hertz in 1887. The classical wave - theory of light could not explain the Photo – electric effect. The failure of classical physics led Max Planck to introduce the Particle-theory (Quantum Physics) of energy. This concept stated that energy is not continuous but discrete. Note: Non-metals also show photoelectric effect. Liquids and gases also show this effect but to limited extent.
- 3. Schematic Diagram of Photoelectric Effect Set Up Apparatus Description : An evacuated tube has two electrodes connected to an external circuit. The metal plate whose surface is to be irradiated acts as the anode. Some of the photoelectrons that emerge from the radiated surface have sufficient energy to reach the cathode despite its negative polarity and they Apparatus Description cathode despite its negative polarity and they constitute the current. As the retarding potential is increased fewer and fewer electrons are able to reach the cathode and the current drops. When V exceeds a certain value V0 no further electrons are able to strike the cathode and the current drops to zero.
- 4. Applications of Photoelectric Effect 1. Automatic fire alarm 2. Automatic burglar alarm 3. Scanners in Television transmission 4. Reproduction of sound in cinema film 5. In paper industry to measure the thickness of paper 6. To locate flaws or holes in the finished goods 7. In astronomy 8. To determine opacity of solids and liquids 9. Automatic switching of street lights 10. To control the temperature of furnace 11. Photometry 12. Beauty meter – To measure the fair complexion of skin 13. Light meters used in cinema industry to check the light 14. Photoelectric sorting 15. Photo counting 16. Meterology
- 5. Laws of Photoelectric Emission Electrons are emitted instantly . Photocurrent is directly proportional to light intensity. There were no photocurrent if the frequency of incident light is less than Threshold Frequency. The maximum kinetic energy of the photoelectrons is directly proportional to the frequency (above the threshold frequency) and independent if light intensity. The Stopping Potential is independent of light intensity and depend on frequency. The number of photoelectrons emitted per second (i.e. photoelectric current) is directly proportional to the intensity of incident light provided the frequency is above the threshold frequency. The process of photoelectric emission is instantaneous. i.e. as soon as the photon of suitable frequency falls on the substance, it emits photoelectrons. The photoelectric emission is one-to-one. i.e. for every photon of suitable frequency one electron is emitted. Work Function (W) In a metal electrons are bound by attraction force of nucleus. So, a certain minimum energy is require to pull out the electron from the surface of metal. It is called WorkFunction of the metal. Work function is a characteristic property of metal. The unit of work function is ‘eV’. 1 eV = 1.6* 10^-19
- 6. Experimental Set-up to study Photoelectric effect ‘V’: Potential applied between Emitter (C) and collecter (A). When Ais cennected to (+ve) and C is (–ve), it is called Accelerating potential and reverse is known as Retarding potential .
- 7. Let us study an experimental setup as shown in the figure. Here a photosensitive C plate is irradiated with light of particular frequency. The plate A is maintained at some potential difference with plate C. When light of suitable frequency falls on the metallic cathode, photoelectrons are emitted. These photoelectrons are attracted towards the +ve anode and hence photoelectric current is constituted. Einstein’s Photoelectric Equation When a photon of energy hν falls on a metal surface, the energy of the photon is absorbed by the electron and is used in two ways: i) A part of energy is used to overcome the surface barrier and come out of the metal surface. This part of the energy is called ‘work function’ ( Ф = h ν 0). ii) The remaining part of the energy is used in giving a velocity ‘v’ to the emitted photoelectron. This is equal to the maximum kinetic energy of the photoelectrons ( ½ mv 2 max ) where ‘m’ is mass of the photoelectron.
- 8. Observations of Photo-electric Effect
- 10. CONCLUSION The photo electric effects was one of the first solid examples of the quantization of light. Because of the result of the metal only releasing electrons from light with high enough frequency, and not just from a lot of lower frequency light, I was concluded that light was quantized. Even a ton of lower energy photons could not trigger the photo electric effect, where as one high energy photon could.