1. Modern Physics 2

2. Dalton’s Atomic Theory 5 points 1)Elements are made of atoms 2)All atoms of an element are identical. 3)The atoms of different elements can be distinguished by their relative weights. 4)Atoms can combine to form a chemical compound 5)Atoms can’t be created or destroyed

3. Blackbody Radiation If an object is hotter than its surroundings it will cool by giving off light. By looking at this light we can determine how hot an object is.

4. Photons Photon – basic unit of electromagnetic radiation Photons were first postulated by Planck Einstein told us that photons have no mass

5. Formulas E = hf E = hc/ λ E = pc p = h/λ

6. Example 1 A 3 milliwatt pen laser radiates at 633nm. Find the values of the following: a) Frequency of light emitted, b) energy of a single photon in joules, c) momentum of a single photon,

7. Photoelectric Effect Light shines on a metal plate and electrons are emitted from this plate and can complete a circuit.

8. Characteristics Current is directly proportional to the light intensity. The current appears without delay when the light is applied. Electrons are emitted only if the light frequency exceeds a threshold frequency.

9. Each electron can absorb a single photon When you increase the intensity of light more photons are created and liberate more electrons

10. “work function” Φ - energy needed to free the electron from the surface Once electron is released from the surface, remaining energy shows up as kinetic energy. Formula E = KE + Φ hf = KE + Φ

11. X-Rays In 1905 we found that electrons can absorb photons, can the opposite happen? Can electrons emit photons?

12. x-rays are the highest energy photons that are associated with electron emission In 1895 Wilhelm Roentgen generated the first man made x-rays

13. Inside the machine is an x-ray tube. An electron gun inside the tube shoots high energy electrons at a target made of heavy atoms, such as tungsten. X-rays come out because of atomic processes induced by the energetic electrons shot at the target.atomic processes

14. Once an electron has gained all its kinetic energy, it can only slow down by emitting photons.

15. When electrons slam into their tungsten target, they give up most of their KE very quickly. Ideally a single photon is emitted to produce the highest energy and smallest wavelength

16. Formula eV = hf eV = hc/λ

17. Example 2 A dental x-ray machine has been set to 120kV at 12mA. Assume that a minimum number of photons are emitted in a 4usec burst from the x-ray machine. a) what is the wavelength for the x-rays produced? B) how many x-ray photons were generated?

18. Wave-Particle Duality Particle – confined to a very small, finite space and has one position at any single value of time Wave – distribute energy and position over many points in space as the same instant

20. Light behaves as both At the radio and microwaves end of the electromagnetic spectrum, the wavelength is too large to observe any particle nature However…

21. At the x-ray and gamma ray end, the wavelength is so small that the energy is focused in a very small region, like a particle Remember, an x-ray is on the order of the size of an atom and gamma rays about the size of a nucleus.

22. In the early 1920’s Louis de Broglie proposed that wave-particle duality could be applied to matter

23. In 1927, Davisson and Germer performed an experiment that validated de Broglie’s idea. Beams of electrons and neutrons were directed at nickel crystals and diffraction patterns were seen.

24. Electrons exhibit the same kind of interference pattern as light does when they’re incident on a double slit.

25. Large masses will have very small wavelengths that are hardly noticeable, so their particle nature will dominate.

26. When a small mass is considered, like an electron, its wave and particle properties must both be considered.