1 / 33

Quantum Theory

Quantum Theory. Classical physics failed to explain certain events which led to the development of the field known as quantum mechanics. Explains particle behavior at the microscopic level These effects of this particle behavior is easily observed

devin-orr
Télécharger la présentation

Quantum Theory

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Quantum Theory • Classical physics failed to explain certain events which led to the development of the field known as quantum mechanics. • Explains particle behavior at the microscopic level • These effects of this particle behavior is easily observed • Ex: Neon signs, chemical flame tests, fireworks, why stars shine, MRI’s, northern lights

  2. History • Maxwell’s wave theory (1860) explained how electric and magnetic fields combine to form an electromagnetic wave. • Maxwell also determined that the speed at which waves move is the speed of light. • Hertz (1887) demonstrated experimentally that Maxwell’s theory was correct.

  3. Unexplained Phenomena • Two unexplained events in the study of optics led to the creation of quantum theory. • Why does molten metal emit light? (hotbody or blackbody radiation) • Why does UV light discharge electrically charged metal plates? (photoelectric effect)

  4. Blackbody Radiation • All objects emit EM radiation • Continuous distribution of wavelengths from infrared to UV (low T to high T) • Coolest flame is __________ • Hottest flame is __________ • A blackbody is an ideal system that absorbs all incoming radiation (they do not exist in nature)

  5. EM Spectrum

  6. Blackbody Radiation Data collected experimentally for the radiation given off by an object at three different temperatures could not be explained by Maxwell’s wave theory

  7. Approximated Blackbody Although it is not truly a blackbody nor ideal, it allows for study of the phenomena. A small hole leading to a cavity inside a block of material. Only a tiny fraction of the radiation can make its way back out of the hole.

  8. Classical Theory - Ultraviolet Catastrophe Classical physics predicted that as the wavelength approaches zero, the amt of E radiated should become infinite Experimentally it was shown that as the wavelength approaches zero, the amount of energy radiated also approaches zero

  9. Quantization of Energy • The failure of classical electromagnetic theory led Max Planck to introduce the idea that energy is not continuous • Planck’s work was based on the idea that thermal energies can be discrete rather than continuous • Energy of vibration E = nhf • n = an integer (0,1,2) • f = frequency of the vibration of the atom • h = 6.63 x 10-34 J•s (Planck’s constant) • hf is called a quantum of energy • The energy of an atom can change only by the absorption or emission of energy in discrete or quantum amounts

  10. Quanta of Light • Einstein introduced the idea that light was quantized in 1905 • He proposed that the atom must emit a specific amount (quantum) of light energy which he named the photon • Each photon has a definite amount of energy that depends on the frequency of light according to E=hf • This idea suggests that light can behave as discrete quanta or “particles” of energy rather than as a wave. • We are able to calculate the energy of each photon using the mathematical equation E = hc/λ

  11. Photoelectric effect • Einstein used the photon concept to explain the photoelectric effect. • Certain metallic materials are photosensitive, meaning that when light strikes their surface, electrons may be emitted. • Released electrons are called photoelectrons. • Negatively charged zinc plates will discharge in the presence of UV radiation, but not visible light or if the plate is positively charged • Led to further investigation where it was discovered that electrons are only ejected if the frequency of radiation is above a certain minimum value called the threshold frequency (ft) • The threshold varies with the metal involved • Ex: Cs – all wavelengths of visible light except red Zn – no wavelength of visible light

  12. Demonstration Videos • Simple Photoelectric Effect Demo - YouTube • photoelectric - YouTube

  13. Characteristics of the Photoelectric Effect • The photocurrent is proportional to the intensity of the light. • The maximum kinetic energy of the emitted electrons depends on the frequency of the light, but not on its intensity. • No photoemission occurs for light with a frequency below a certain cutoff frequency fο, regardless of the light intensity. • A photocurrent is observed immediately when the light frequency is greater than fο, even if the light intensity is extremely low. Only #1 is predicted by wave theory. Einstein’s quantum theory of light and his photon model of light explain ALL experimental results of the photoelectric effect.

  14. Applications of the photoelectric effect • Photography light meters • Photocells • Electric eye – garage door safety mechanism

  15. Compton Effect • 1923 Arthur Compton conducted experiments with X-rays and graphite and explained the observed results (the scattering of X-rays from a graphite block) by assuming the radiation to be composed of quanta • His explanation of the observed effect provided additional convincing evidence that light energy is carried by photons. • Idea was that if light is a particle, when it interacts with other particles, the two should act like pool balls • Results showed scattered rays with less energy (momentum) and longer wavelengths than incoming rays; and electrons gained energy (momentum) and had shorter wavelengths • The changes in the two particles were exactly equivalent • Only significant for X-rays and gamma-ray scattering

  16. Compton Shift The blue ball is the recoiling electron with increased energy and shorter wavelength The blue squiggle is the scattered ray with decreased energy and longer wavelength The energy of the two combined is equivalent to the incident ray

  17. Dueling theories • Classical wave theory says that light is a traveling wave, and this satisfies such phenomena as interference and diffraction. • Quantum theory, with says light can be a particle is necessary to explain the photoelectric and Compton effects. • The two were combined to produce a description of the dual nature of light.

  18. Wave Particle Duality • To explain all electromagnetic phenomena, light has to be considered sometimes as a wave and other times as a beam of photons. When it interacts with small (quantized systems), such as atoms, nuclei, and molecules, the photon (quantized energy) picture must be used. In everyday-sized systems – for example, slit systems causing diffraction and interference – the wave model is applicable.

  19. Contributors to Wave-Particle Duality • deBroglie proposed in 1923 that particles have wave properties • This led to changes in the atomic model of the atom b/c electrons were no longer thought to be in specific orbit around the nucleus, they were moving with a wave-motion instead • This led directly to Heisenberg developing the uncertainty principle (can’t know both the _____________ & ___________ ) why?

  20. Momentum of photons • The photoelectric effect demonstrated that photons have no mass – just energy • Einstein (1916) predicted that the photon should have another property of particles: momentum • The Compton effect verified that photons have momentum • Formula for photon momentum is • p = hf = h c 

  21. Light emission and absorption • Differences between incandescent light and gas-discharge tubes became apparent • Incandescent = continuous spectrum (all wavelengths are present) • Gas-discharge tubes = emission spectrum – (which means discrete spectra with only specific wavelengths were observed) • Atoms can absorb light as well – which produces a dark-line spectrum or absorption spectrum.

  22. Atomic Emission Spectrum

  23. Absorption Spectrum

  24. Emission Spectrum • Early 1900’s, it was observed that certain elements emitted visible light from the sun, microwaves and x-rays • Analysis revealed that an element’s chemical behavior is related to the arrangement of it’s electrons • Each element has its own unique emission spectrum, which allows for identification of elements using spectroscopic analysis

  25. The Bohr Theory of the Hydrogen Atom • Niels Bohr provided an explanation of the spectral lines in 1913. • He proposed that the attractive force between the protons in the nucleus and the electrons outside supplied the necessary centripetal force for the electron to stay in orbit around the nucleus. • The flaw in this initial explanation was that classical theory indicates the electron should be emitting light due to acceleration, which it was not doing.

  26. The Bohr Theory of the Hydrogen Atom • Bohr further postulated that the electrons do not emit light when in a stable discrete orbit, only when they change energy levels. • When electrons make a downward transition to an orbit of lower energy level, light is emitted. • When electrons make an upward transition to an orbit of higher energy level, light is absorbed.

  27. Energy Levels • Orbits of electrons are commonly referred to by their energy level. • Ground state is the lowest allowable energy level (principal quantum number 1) • Excited states are those levels above ground state. • Electrons are normally located in the ground state, but when excited, they move up energy levels by absorbing discrete quantities of energy (ladder rungs) • Every jump from one energy level to another corresponds to a specific spectral line (either emission or absorption)

  28. Energy Level Diagram of Hydrogen

  29. The Bohr Theory of the Hydrogen Atom • The Bohr model worked well for hydrogen and an other atoms that had only one electron (ionized helium), but could not successfully describe multi-electron atoms. • His theory produced fragments of quantum theory into a basically classical framework

  30. LASER • Light Amplification by Stimulated Emission of Radiation • Based on electrons being excited • Normally electrons have very short lifetimes in their excited state • Some materials have relatively long lifetimes (called a metastable state) and are phosphorescent (glow in the dark materials) and can emit light for lengthy periods of time.

  31. Stimulated Emission (Einstein…again) • When a photon with energy equal to an allowed transition (from ground state to an excited state or b/w 1st and 2nd excited state…) strikes an atom already in a metastable state, it may stimulate that atom to make a transition to a lower energy level, which is called stimulated emission. • This transition yields a photon identical to the original photon, thus two photons are able to go in the next round (1 in and 2 out each time) allows the reaction to be amplified.

  32. Uses of Lasers • Helium-Neon lasers: characteristic red light • Check-out scanners • CD/DVD • Other lasers • Varicose veins • Tattoo removal • Holographic images

More Related