Quantum - Photons

Photons

Electromagnetic waves are emitted in packets called 'quanta', the energy of each quantum is given by:E = hf    or E = hc / fE = energy in joulesf = frequency in hertzh = Planck's constant = 6.63x10^-34A quantum of light is called a photon.

The Electron Volt

The electron volt is the energy gained by one electron when it is accelerated through a p.d. of one volt.1eV = 1.6X10^-19J

Photoelectric Effect

Electrons are ejected from metal surfaces when EM radiation of a high enough frequency falls on them.For a given surface there is a minimum frequency of light which causes photoelectric emission.The number of photo-electrons emitted per second depends on the intensity of the light but not on the frequency.The maximum kinetic energy of the photo electrons increases as the frequency increases and it is independent of intensity.

Wave theory cannot explain why: A threshold frequency exists and why it is different for different metals. The rate of emission is dependent on intensity but not frequency. Prompt emission occurs.

When a photon of light strikes an electron, its energy may be transferred to the electron, which may have enough energy to escape from the metal surface.A photon cannot share its energy with more than one electron.An electron cannot gain its energy from more than one photon.If an electron needs a certain amount of energy to become free of the atom, then there is a minimum energy required, called the work function Φ.Therefore there must be a minimum frequency, since E = hf. If E

Einstein's Photoelectric Equation:He proposed that the energy of the photon:- overcame the binding energy on the electronAND- provided kinetic energy for the electron to move away from the nucleus h f = Φ + EkΦ - the minimum energy required to release the electron, known as the work function.Ek - maximum kinetic energy = 1/2(mv^2) of the emitted photo 

So all the energy from one photon is given to one electron, part of the photon's energy overcomes the work function, any remaining is converted into Ek of the emitted photo electron.

Ek = h f  -  Φy  = m x + cPlotting a graph of energy against frequency, Planck's constant, h, is the gradient.Negative y intercept is the work function.

Practical Use: A photoelectric cellWhen electromagnetic radiation, of frequency above the threshold frequency, falls on the photoelectric cell then photo-electrons are released. These flow due to the p.d. of the supply cell and a current is registered in the micro-ammeter.If the intensity is doubled then twice as many photons arrive per second, so twice as many electrons are released per second.As current is the rate of flow of charge, then current doubles.This assuming that one photon can only release one electron and that all photo electrons emitted are collected.

Wave Particle Duality

Wave like properties:- Reflection- Refraction- Diffraction- InterferenceParticle like properties:- Collisions- Deflection in electric and magnetic fieldsλ = h / mvm = mass of the particlev = the velocity of the particleλ = the de Broglie wavelength

Evidence of Electron Diffraction: Produces rings on the screen ->Electrons, protons, neutrons and alpha-particles will all produce a similar pattern. Alpha particles are the least favourite particle as they are larger, have a greater mass and therefore by de Broglie's equation will have a smaller wavelength and so are more difficult to detect.

Photons