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Physics 2

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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

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 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 10-34 Js

- 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.

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

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

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

- 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 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

- 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

photon E = energy to remove electron (work fn) + max KE of ejected electron

hf = hft + KEmax

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)

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.

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

- 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.