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Quantum Physics

Quantum Physics. Quantum physics is a discipline that attempts to explain the behavior of matter at the atom level.

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Quantum Physics

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  1. Quantum Physics • Quantum physics is a discipline that attempts to explain the behavior of matter at the atom level. • Such behavior includes but not limited to Blackbody Radiation, electromagnetic radiation by heated objects, Photoelectric effect, emission of electrons from illuminated metals and the emission of sharp spectral lines from gas atoms in an electric discharge tube.

  2. Blackbody Radiation • A blackbody is an ideal system that absorbs all radiation incident upon it. • An object at any temperature emits electromagnetic radiation as thermal radiation. At low temperatures the radiation is in the infrared region, not visible. At higher temperatures objects will glow red and at higher temperatures eventually white. Ex. The light bulb.

  3. Cont. • The emitted radiation originates from accelerated charges near the surface of the object and have a distribution of frequencies producing a continuous spectrum. • Planck hypothesized that blackbody radiation was caused by submicroscopic charged oscillators (resonators) that had discrete energies. These energies were quantized. Where En = nhf n= quantum number, f = frequency of vibration h Planck’s constant = 6.626x10-34 J.s

  4. Photoelectric Effect • Intense light incident on certain metal surfaces causes emission of electrons called photoelectrons. (Hertz). • Several features can not be explained by classic physics. 1) If the incident light falls below a cutoff frequency (fc) no electrons are emitted. 2)The maximum kinetic energy of the photoelectrons is independent of the light intensity

  5. Cont. 3)The maximum kinetic energy increases with increase in light frequency 4) Electrons are emitted almost instantly after the surface is illuminated. • Einstein suggested that a photon is created when an oscillator moves down a quantum level where E = hf. As this energy is localized in a very small package it give all its energy to an electron upon contact with the metal.

  6. Cont. • The maximum kinetic energy of the photoelectrons is given by Kemax = hf -  Where  = work function of the metal, the minimum energy binding the electron to the metal. 1’) The energy of a photon must be at least equal to the work function explaining cutoff 2’) KE max depends on frequency so intensity is immaterial .

  7. Cont. 3’) KEmax is linear with frequency so KE increases with increase in frequency. 4’)Electrons are emitted instantly because light energy is in packages rather than waves. • AS f = v/ c = hc/ • Wavelengths greater than c do not result in the emission of photoelectrons.

  8. X-Rays • X-rays are electromagnetic waves of very short wavelength. ( around 0.1 nm) They travel at a speed near that of light and they are not deflected by either electric or magnetic fields. ( they have no charge). • X-rays are produced when high speed electrons are suddenly slowed down.

  9. Diffraction of X-rays • Because x-rays are of such short wavelength and a diffracting grating must be of the order of the wavelength, it is not possible to construct a grating. Accordingly, because ionic crystals have an orderly lattice formation in parallel planes they can be used as a diffracting grating. • X-ray diffraction is used to investigate molecular structures

  10. The Compton Effect • Arthur Compton in 1923 noted that the wavelength of incident x-ray increased after being scattered by contact reflection with graphite. • Compton proposed that the incident photon upon colliding with the electron and dislodging it passed some of its energy onto the electron, resulting in an decrease in its energy and frequency increasing wavelength.

  11. Cont. • The change in wavelength called the Compton shift is given by Δλ where Δλ = λ – λo = (h/mec)(1-cosθ) where me = mass of the electron θ = the angle between the scattered and incident photons h/mec = Compton wavelength = 0.00243nm

  12. The Dual Nature of Light and Matter • The photoelectric effect offers evidence that light behaves like a particle, having energy hf and momentum h/. Interference and diffraction patterns indicate that light behaves like a wave. • Therefore light has a dual nature exhibiting both particle and wave characteristics.

  13. De Broglie Wavelength • French physicist De Broglie proposed that all matter exhibits particle wave characteristics. This was later demonstrated by experiments by Davisson and Germer. • The wavelength of a particle is given by  = h/mv • The frequency being f = E/h

  14. Heisenberg Uncertainty Principle • If one measures the position and speed of a particle at an instant there will be experimental uncertainty in the measurement. • To view an electron a photon of light must bounce of the electron and be detected by the eye. When the photon strikes the electron it transfers some momentum to the electron thus shifting its position making that position uncertain. x px  h/4

  15. Assignment • Write a brief about • 1) The electron microscope • 2) The scanning tunneling microscope.

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