Origin of Quantum Theory

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Presentation transcript:

Origin of Quantum Theory Black Body Radiation Photoelectric Effect Compton Scattering

Who first postulated the idea of light quanta? 1. Planck 2. Bohr 3. De Broglie 4. Einstein 5. Heisenberg

Who first postulated the idea of light quanta? 1. Planck 2. Bohr 3. De Broglie 4. Einstein 5. Heisenberg

In 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.

In 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.

The 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.

The 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.

En = nhf Blackbody Radiation Planck (1900) Compton scattering Compton (1923) En = nhf Photoelectric Effect A. Einstein (1905) Bohr Atom N. Bohr (1911)

Introduction to Radiation Hot Solid Hot Gas Cold Gas

Spectra Absorption spectra Emission or Line spectra

This is one way we identify interstellar elements.

Blackbody Radiation Demonstration - light bulb filament Demonstration - idea blackbody What happens when you turn on the electric bunner on a stove?

How was the spectra measured?

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. power area area under curve

T = ________ lmax = ________ lmax T = ________ 6,000 K 5,000 K 7,000 K 425 nm 485 nm 3.0X10-3 mK 600 nm What do you notice about these values? They all have the same value.

Classical Theory (Maxwell’s Electromagnetic Theory) Planck’s Theory (Quanta) ultraviolet catastrophe

Photoelectric Effect

E0 is the work function or binding energy

The work function of metal A is 3. 0 eV The 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

The work function of metal A is 3. 0 eV The 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

The 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.

The 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.

Compton Scattering

Compton Scattering

Classical Scattering of Electromagnetic Wave We do not have to consider the magnetic field since it is so small compared to the electric field So the radiation wavelength is the same as the incident wavelength.

Compton Scattering 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

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 energy Conservation of momentum

Write the energy and momentum for each before and after the collision Conservation of Energy Conservation of Momentum

Write the energy and momentum for each before and after the collision before after electron photon ?

Introduction to Radiation Solid Hot Gas Cold Gas

Planck’s Theory (Quanta) Classical expression

Planck’s Theory (Quanta)

Optical Pyrometer

Microwave Cosmic Background

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?