# Physics 2 Chapter 27 Sections 1-3.

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Physics 2 Chapter 27 Sections 1-3

All objects emit EM radiation Usually consists of a continuous distribution of λ As temp increases the more the max intensity shifts to shorter λ As temp increases the total energy given off by the body (area under curve) increases

Classical physics predicts that as λ approaches 0 the amount of energy radiated should become infinite Instead data shows that as λ approaches 0 the amount of energy radiated also approaches 0 1900 Max Planck developed a formula for blackbody radiation that matched the experimental data

Blackbody Radiation Blackbody is a hollow object with a small opening through which light can enter Energy gets absorbed every time light hits the wall inside If box is insulated the energy absorbed causes the temp inside to rise Energy will be radiated inside the box and some will escape

Planck’s Theory Planck said a blackbody is made up of a large number of atomic oscillators, called resonators, which emit EM radiation of various frequencies Resonators can only absorb and give off energy in discrete amounts given by E = nhf where n=quantum # (0,1,2…), f=frequency of resonator, h=Planck’s constant = 6.63 x Js

Planck’s Theory Energy is quantized
Discrete units of light energy are called quanta Resonator will only radiate/absorb energy when it changes quantum states The idea was so radical that even Planck didn’t think it was realistic. It was just a mathematical model.

Photoelectric Effect Phenomenon where electrons are ejected from a metal plate when light of certain frequencies shines on it

Photoelectric Effect Classical physics says:
When light waves of any f strike the metal they should eject electrons if their intensity is high enough At low intensity, electrons can be ejected if wait long enough for electrons to absorb the incoming energy Increasing the intensity increases the KE of the ejected electrons None of these are true!

Photoelectric Effect Observations:
No electrons are emitted if frequency is below a certain level, regardless of intensity If frequency exceeds this threshold frequency the number of electrons emitted is proportional to the intensity KE of emitted electrons is independent of intensity KE increases as frequency increases Electrons are emitted almost instantly, even at low intensity 1905 Einstein used Planck’s quantum idea to explain the photoelectric effect

Einstein’s Theory Photon – discrete unit of light energy
EM waves are composed of photons Each photon has an energy, E, given by E = nhf that can be absorbed by an electron

Einstein’s Theory Einstein says that if light hits a metal it can do 1 of 2 things: Photon can give its energy to an electron in the metal Photon can do work of removing the electron from the metal

Einstein’s Theory Work function – minimum amount of energy required for an electron to escape from a metal, hft, where ft is threshold frequency If photon has more energy than hft the rest goes into KE of ejected electron

Einstein’s Theory photon E = energy to remove electron (work fn) + max KE of ejected electron hf = hft + KEmax

Einstein’s Theory If f < ft then no ejected electrons
KEmax ft f If f < ft then no ejected electrons KE only depends on f not intensity (increase in intensity means more ejected electrons but at same KE) Slope is h (Planck’s constant) Einstein’s photon model of the photoelectric effect didn’t achieve much acceptance until 1923 and Arthur Compton

Sample Problem The work function for a silver surface is 4.73 eV. Find the minimum frequency that light must have in order to eject electrons from its surface. 1.14 x 1015 Hz

Compton Effect or Shift
Compton struck an electron at rest with an xray photon Noticed scattered photon has a frequency that is less than the incident frequency Difference between the 2 frequencies depended on the angle at which the scattered photon left

Compton Effect or Shift
Phenomenon in which a photon is scattered with a smaller frequency or larger wavelength than the incident photon This phenomenon became the basis for the idea of wave-particle duality.