Prozorov_35 - PHYSICS 222 Introduction to Classical Physics...

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Unformatted text preview: PHYSICS 222 Introduction to Classical Physics II Prof. Ruslan Prozorov Iowa State University Fall 2011 LECTURES 35 Photoelectric effect. Atomic spectra. Black body radiation. The photoelectric effect PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 2 The photoelectric effect A photon of blue light striking a sample of Cs will knock off an electron while a photon of red light will not. Why? The energy of a blue photon exceeds the ionization energy of Cs outermost electron. When a blue photon strikes a Cs sample, an electron escapes, taking the extra energy as kinetic energy of motion. PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 3 Einstein’s explanation A photon contains a discrete amount of energy. This energy may be calculated for any type of EM radiation using E = h or E = (hc)/λ, where h is Plank’s constant h=6.626 × 10−34Js. kinetic energy of an electron: K h W W hf 0 work function threshold frequency PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 4 stopping potential • If the frequency and the intensity of the incident radiation are fixed, the photoelectric current increases gradually with an increase in positive potential until all the photoelectrons emitted are collected. The photoelectric current attains a saturation value and does not increase further for any increase in the positive potential. The saturation current depends on the intensity of illumination, but not its wavelength. • If we apply a negative potential to a plate that collects the electrons emitted by another (illuminated) plate and gradually increase it, the photoelectric current decreases until it is zero, at a certain negative potential. This is called stopping potential or cut off potential. • for the given frequency of incident radiation, the stopping potential is independent of its intensity. • For a given frequency of the incident radiation, the stopping potential Vo is related to the maximum kinetic energy of the photoelectron that is just stopped from reaching the plate PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 5 The photoelectric effect PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 6 spectroscopy continuous spectrum emission spectrum absorption spectrum PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 7 atomic line spectra A pure atomic sample can be sealed in a glass tube and heated by electrical discharge. The light emitted when atoms excited by the electrical energy is released takes the form of distinctly colored lines. These lines will always form the same pattern for a given atom, which is very unlike the behavior of white light. PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 8 atomic line spectra As an electron moves from an excited state to its ground state, a photon of certain energy is released. PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 9 hydrogen PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 10 hydrogen spectra energy required to remove an electron to infinity! PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 11 forbidden and allowed transitions sodium’s atomic spectrum has states that can be excited by a flame which don’t readily decay to the ground state (forbidden). Those that do occur (allowed) are identified by their wavelength. In a centrosymmetric environment transitions between like atomic orbitals such as s-s,p-p, d-d, or f-f transitions are forbidden. PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 12 Spectral transitions PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 13 the “nuclear” atom Rutherford did a very clever experiment with thin Au foil and alpha particles. Scattering was nearly absent or very dramatic, leading him to conclude that the atom was mostly empty space around a dense (+) center. PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 14 The Bohr model Classical physics predicts that a moving electron would emit radiation and its orbit will decay the orbits are QUANTIZED! PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 15 Hydrogen-like atoms Solutions for H apply to He+ because everything hinges on the system being a “two-body problem.” Li++ works also, but soon things become overly artificial. PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 16 The laser Light amplification by stimulated emission of radiation is what the acronym stands for. Different excited states will naturally radiate on a timetable appropriate for their states, but a light wave passing an excited state increases the likelihood of the emission at its particular frequency. PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 17 HeNe laser Small semiconducting lasers have sold many millions since the inception of CD and DVD devices among personal electronic equipment, but the HeNe laser has filled many laser pointers. PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 18 laser pointer The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode (LED). PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 19 x-rays and scattering An experimental arrangement for making. Information about the quantum nature of x-rays first came from Compton scattering in 1923. PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 20 Compton experiment PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 21 blackbody radiation o A blackbody is easily thought of by watching the temperature and color change of an electrical stovetop burner. o All was well with blackbody radiation until Raleigh tried to use the system to generate curves to include the blue, violet, and ultraviolet regions of the visible spectrum. This failure of classical predictions to correctly pose any explanation of this effect was one of the first clues leading to the quantum theory. PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 22 ultraviolet catastrophe PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 23 the ultraviolet catastrophe • The ultraviolet catastrophe results from the equipartition theorem of classical statistical mechanics which states that all harmonic oscillator modes of a system at equilibrium have an average energy of kT / 2. • most of the energy in a natural vibrator will be in the smaller wavelengths and higher frequencies, where most of the modes are. • According to classical electromagnetism, the number of electromagnetic modes in a 3-dimensional cavity, per unit frequency, is proportional to the square of the frequency. • Max Planck solved the problem by postulating that electromagnetic energy did not follow the classical description, but could only be emitted in discrete packets of energy proportional to the frequency, as given by Planck's law. • This has the effect of reducing the number of possible modes with a given energy at high frequencies • The formula for the radiated power for the idealized system (black body) was in line with known experiments, and came to be called Planck's law of black body radiation. • Based on past experiments, Planck was also able to determine the value of its parameter, now called Planck's constant. • The packets of energy later came to be called photons PHYS222 - Lecture 35 - Prof. Ruslan Prozorov - Iowa State University 16 November 2011 24 ...
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