Origin of Quantum Theory Black Body Radiation Photoelectric Effect Compton Scattering

Origin of Quantum Theory

Black Body Radiation

Photoelectric Effect

Compton Scattering

Origins of Quantum TheoryWho first postulated the idea of light quanta?

1. Planck2. Bohr3. De Broglie4. Einstein5. Heisenberg

Origins of Quantum TheoryWho first postulated the idea of light quanta?

1. Planck2. Bohr3. De Broglie4. Einstein5. Heisenberg

Origins of Quantum TheoryIn the photoelectric effect experiment, current flows when the light frequency is

1. less then the threshold frequency.2. equal to the threshold frequency.3. greater then the threshold frequency.4. less than the cathode’s work function.5. equal to the cathode’s work function.

Origins of Quantum TheoryIn the photoelectric effect experiment, current flows when the light frequency is

1. less then the threshold frequency.2. equal to the threshold frequency.3. greater then the threshold frequency.4. less than the cathode’s work function.5. equal to the cathode’s work function.

Origins of Quantum TheoryThe minimum amount of energy needed to free an electron from a piece of metal is called the

1. Gibb’s free energy.2. quantum energy.3. liberation potential.4. threshold energy.5. work function.

Origins of Quantum TheoryThe minimum amount of energy needed to free an electron from a piece of metal is called the

1. Gibb’s free energy.2. quantum energy.3. liberation potential.4. threshold energy.5. work function.

Origins of Quantum Theory

Blackbody RadiationPlanck(1900)

En = nhf

Photoelectric EffectA. Einstein

(1905)Bohr Atom

N. Bohr(1911)

Compton scatteringCompton(1923)

Origins of Quantum Theory Introduction to Radiation

Hot Solid

Hot Gas

Cold Gas

Origins of Quantum Theory Spectra

Emission or Line spectra

Absorption spectra

Origins of Quantum Theory This is one way we identify interstellar elements.


What happens when you turn on the electric bunner on a stove?

Blackbody Radiation

Demonstration - light bulb filament Demonstration - idea blackbody

Origins of Quantum Theory How was the spectra measured?

Origins of Quantum Theory Here is what they knew at the turn of the century.

The total power radiated is

The wavelength for maximum radiation is inversely proportional to the temperature

The distribution of wavelengths depends

on the temperature only, not the material.

maxT 2.898X10 3 mK

R ,T

AP

I power

area

Pblackbody AT 4

5.67X10 8 W m2K 4

area under curve

Origins of Quantum TheoryT = ________ max = ________

max T = ________

T = ________ max = ________

max T = ________

T = ________ max = ________

max T = ________

What do you notice about these values?

6,000 K

5,000 K

7,000 K 425 nm

485 nm

3.0X10-3 mK

3.0X10-3 mK

3.0X10-3 mK

600 nm

They all have the same value.

maxT 2.898X10 3 mK


Origins of Quantum Theory Classical Theory (Maxwell’s Electromagnetic Theory)

Planck’s Theory (Quanta)

E nhf

ultraviolet catastrophe

12

),(5

2

kThcehc

TR

R(,T) 2ckT

4

Origins of Quantum Theory Photoelectric Effect


Ebefore Eafter

eKEEhf 0

stoppinge eVKE

fe

h

e

EVstopping 0

E0 is the work function or binding energy













Origins of Quantum TheoryThe work function of metal A is 3.0 eV. Metals B and C have work functions of 4.0 eV and 5.0 eV, respectively. Ultraviolet light shines on all three metals, creating photoelectrons. Rank in order, from largest to smallest, the stopping voltages for A, B, and C.

1. VA > VB > VC 2. VC > VB > VA 3. VA = VB = VC

Origins of Quantum TheoryThe work function of metal A is 3.0 eV. Metals B and C have work functions of 4.0 eV and 5.0 eV, respectively. Ultraviolet light shines on all three metals, creating photoelectrons. Rank in order, from largest to smallest, the stopping voltages for A, B, and C.

1. VA > VB > VC 2. VC > VB > VA 3. VA = VB = VC

Origins of Quantum TheoryThe intensity of a beam of light is increased but the light’s frequency is unchanged. Which of the following is true?

1. The photons travel faster.2. Each photon has more energy.3. There are more photons per second. 4. The photons are larger.

Origins of Quantum TheoryThe intensity of a beam of light is increased but the light’s frequency is unchanged. Which of the following is true?

1. The photons travel faster.2. Each photon has more energy.3. There are more photons per second. 4. The photons are larger.

Origins of Quantum Theory Compton Scattering

scattered incident h

mc1 cos

scattered

incident



mc1 cos

Origins of Quantum Theory Classical Scattering of Electromagnetic Wave

So the radiation wavelength is the same as the incident wavelength.

We do not have to consider the magnetic field since it is so small compared to the electric field


Answer the first three questions in the tutorial and draw the diagram for the scattering experiment.

Discuss the Compton experiment in your group. What are the two physical quantities in the collision?

What is the assumption for light that Compton used?

That light behaved like a particle with energy

And momentum

E hf

p E

c

hf

c

E2 pc 2 mc

2 2 m 0

Origins of Quantum Theory Compton Scattering What two equations (conservation law) did Compton use to

find the change in wavelength?

Draw the collision between the electron and the photon. Label the angles and .

Conservation of energyConservation of momentum

Origins of Quantum TheoryWrite the energy and momentum for each before and after

the collision

Conservation of Energy

Conservation of Momentum

fefpieip EEEE ,,,,

cos2 ,,

2

,

2

,

2

fpipfpipe ppppp efpip ppp

,,

2222 mccphfcmhf efei

ipp ,

fpp ,

ep

Origins of Quantum TheoryWrite the energy and momentum for each before and after

the collision

before after

electron

photon

E e,i

Ee, f

pe,i

pe, f

E p,i

pp,i

E p, f

pp, f

2cme

222 cmE ee

i

iip hc

hfc

E

,

ihffhf

f

ffp hc

hf

c

E

,

0

?


mc1 cos


Origins of Quantum Theory Introduction to Radiation

Solid

Hot Gas

Cold Gas

Origins of Quantum Theory Planck’s Theory (Quanta)

R(,T) 2hc 2

5 ehc kT 1

kTc

hckThc

kThc

hc

ehc

TRkThc

45

2

5

2

5

2

22

11

2

12

),(

Classical expression

Origins of Quantum Theory Planck’s Theory (Quanta)

R(,T) 2hc 2

5 ehc kT 1

kThc

kThc

kThc

ehc

ehc

ehc

TR

5

2

5

2

5

2

22

12

),(

0

Origins of Quantum Theory Optical Pyrometer

Origins of Quantum Theory Microwave Cosmic Background


Origins of Quantum Theory Use the results above and substitute into the conservation

equation.

Do the same for conservation of momentum in the x direction.

Now for momentum in the y direction.

Compton solved these three equations to find = - . Where is the initial wavelength for the photon and is the final wavelength for the photon.

What is the final results (Compton’s equation) that Compton found?


mc1 cos

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Origin of Quantum Theory Black Body Radiation Photoelectric Effect Compton Scattering