Physics. Light Quanta

Transcription

1 Physics Light Quanta

2 Quantum Theory Is light a WAVE or a PARTICLE? Particle tiny object like a bullet, has mass and travels in straight lines unless a force acts upon it Waves phenomena that extend in space, characterized by diffraction and interference

3 Evidence for Wave Nature Thomas Young s double slit experiment showed interference patterns, demonstrating light s wavelike properties.

4 Evidence for Wave Nature Maxwell reinforced Young s results by predicting that light carries energy in oscillating electric and magnetic fields waves.

5 Black Body Radiation & The UV Catastrophe According to classical wave theory, the intensity should increase as the fourth power of the wavelength decreased. At low wavelengths, then intensity should become infinite! Wave theory could not explain the peak in the intensity.

6 Planck s Constant and Photons In 1900, Maxwell Planck hypothesized that radiant energy was emitted in discrete bundles or quanta, the energy of which is proportional to the frequency of radiation. E ~ f

7 Energy is Proportional to Frequency Microwave radiation can t do the damage to molecules in living cells that UV light and X- Rays can. EM radiation interacts with matter only in discrete bundles of photons. So the relatively low frequency of microwaves ensures low energy per photon.

8 Energy is Proportional to Frequency UV light can deliver about a million times more energy to a molecule because its frequency is a million times greater than the frequency of microwaves.

9 Photoelectric Effect Robert Millikan Albert Einstein

10 Evidence for Particle Nature The Photoelectric Effect

11 Ionization Energy Ionization Energy, also called the work function, ф, is the minimum energy required to eject electrons from the atom and is equal to the absolute value of the ground state energy. K max = E photon φ K max = hf φ

12 Photoelectric Effect

13 Photoelectric Effect 1. Photons with very low frequencies create no potential in the photocell. 2. As the frequency of the photons is steadily increased, a threshold frequency was encountered that induced a potential in the photocell. 3. Increasing the intensity of the light (but using the same frequency) increases the rate at which electrons are ejected, but they still have the same energy (K max ). The voltage doesn t change. 4. Increasing the frequency of the photons above the threshold frequency increased the potential of the photocell and the energy of the emitted electrons.

14 Photoelectric Effect

15 K max vs. Frequency

16 Photoelectric Effect

17 Wave-Particle Duality E = hf Light is emitted / absorbed like a particle, but travels as a wave.

18 Double-Slit Experiment

19 Double-Slit Experiment Suppose we dim our light source, allowing only one photon at a time to pass through and reach the film.

20 Double-Slit Experiment

21 Double-Slit Experiment

22 Double-Slit Experiment

23 Double-Slit Experiment

24 Double-Slit Experiment

25 Light strikes as a particle, but Light travels as a wave!

26 Particles as Waves LIGHT ELECTRONS

27 Particles as Waves Every particle of matter is somehow endowed with a wave to guide it as it travels! Yes, you read that correctly electrons, protons, atoms, a mouse, you, a planet, a star Louis de Broglie They all have a wavelength that is related to it s momentum.

28 Particles as Waves λ = h momentum λ = h mv Louis de Broglie

29 Problems Calculate the wavelength of a bullet of mass 0.02 kg, traveling at 330 m/s.

30 Problems Calculate the wavelength of an electron (mass 9.1 x kg), traveling at 2% the speed of light.

31 Quantum Uncertainty Uncertainty is related to the limits of our ability to make measurements and the idea that the act of measuring something affects the quantity being measured.

32 Quantum Uncertainty The theory says a lot, but does not really bring us any closer to the secret of the old one. I, at any rate, am convinced that He does not throw dice. - Albert Einstein

33 Quantum Uncertainty

34 Observing vs. Probing Quantum uncertainties are significant only in the atomic and subatomic realm.

35 Quantum Uncertainty Let s measure the speed of a pitched baseball using a pair of photogates that are a know distance apart.

36 Quantum Uncertainty Now, let s use the same apparatus to measure the speed of an electron.

37 Quantum Uncertainty

38 The Uncertainty Principle The Heisenberg Uncertainty Principle states that it is not possible to measure exactly both the position and the momentum of a particle at the same time. Werner Heisenberg Δp Δx ħ

39 The Uncertainty Principle If we wish to know the momentum of an electron with great accuracy (small delta p), the corresponding uncertainty in position will be large. If we wish to know the position of an electron with great accuracy, (small delta x), the corresponding uncertainty in momentum will be large.

40 Complementarity Complementarity is the principle stating that the wave and particle aspects of both matter and radiation are necessary, complementary parts of the whole. Niels Bohr

41 Other Interesting Quantum Phenomena Quantum Tunneling Entanglement (Spooky Action at a Distance) Quantum Computers

42 Physics The Atom & The Quantum

43 Electron Waves The idea that electrons can occupy only certain levels was first suggested by Neils Bohr in 1913.

44 Electron Waves Niels Bohr Why did electrons only occupy specific discrete energy states? It seems like an electron should be able to orbit at any distance depending only on it s speed, like any satellite.

45 Electron Waves According to Classical Mechanics, an electron moving in a circular path should continually radiate energy and this loss of energy should cause it to spiral into the nucleus. But this does not happen!

46 Electron Waves λ = h mv Why the electron occupies only discrete levels is understood by considering the electron to be not a particle but a WAVE.

47 Electron Waves An orbiting electron forms a standing wave only when the circumference of its obit is equal to a whole-number multiple of the wavelength.

48 Electron Waves

49 Cloud Model of the Atom In the still more modern wave model of the atom, electrons move not only around the nucleus but also in and out, toward and away from the nucleus.

50 Schrödinger s Wave Equation Erwin Schrödinger

51 Quantum Mechanics Richard Feynman

52 Correspondence Principle The correspondence principle says that in order for a new theory to be valid, it must account for the verified results of the old theory. When the techniques of quantum theory are applied in classical situations, the results are identical.

53 Other QM Topics to Investigate Quantum Entanglement Quantum Tunneling String Theory Superposition