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Chapter 6. Light Source and Detectors

Chapter 6. Light Source and Detectors. Quantum- element units of energy Quantum optics: photoelectric effect laser emission blackbody radiation. 6.1 Light Sources. 1. Light Sources. An object is a source of light.

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Chapter 6. Light Source and Detectors

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  1. Chapter 6. Light Source and Detectors Quantum- element units of energy Quantum optics: photoelectric effect laser emission blackbody radiation

  2. 6.1 Light Sources 1. Light Sources • An object is a source of light. • A direct source produces light, e.g. the sun, light bulb, fire. • An indirect source does not produce light, e.g. an illuminated object. • An extended object may be regarded as a set of point sources.

  3. (a) Thermal source: sun, wax candle, kerosene lanterns, electric light bulb light--the consequence of the temperature • kerosene lanterns: carbon freed by the combustion process • electric light bulbs: a filament is heated. carbon filaments, metal filaments Incandescent lamps:be heated to incandescence  Refractory metals: a high melting point  Tungsten: 3410C ; evaporates,  Some halogens( iodine), retard the process

  4. How tungsten filaments works

  5. (b) Fluorescent lamps • Fluorescent lamps • High-pressure mercury lamps • High-pressure xenon lamps • (c) Stimulated emission: laser, LED

  6. 6.1 Light Sources 2. Blackbody Radiators (a) Black body : is an idealabsorber, also a perfectemitter • A good way of making a blackbody is to force reflected light to make lots of reflections: inside a bottle with a small opening • The spectral distribution of that radiation is a function of temperature alone; the material as such plays no role

  7. Classical theory failed • Ultraviolet catastrophe

  8. Quantization of Energy Max Planck (1858-1947) Solved the “ultraviolet catastrophe” • Planck’s hypothesis: An object can only gain or lose energy by absorbing or emitting radiant energy in QUANTA.

  9. Note: Long wavelength  small frequency Short wavelength high frequency increasing frequency increasing wavelength Electromagnetic Radiation • All waves have: frequency and wavelength • symbol: n (Greek letter “nu”) l (Greek “lambda”) • units: “cycles per sec” = Hertz“distance” (nm)

  10. Energy of radiation is proportional to frequency. E = h • n where h = Planck’s constant = 6.6262 x 10-34 J•s Light with large l (small n) has a small E. Light with a short l (large n) has a large E. (b) Photon: the oscillators emit energy, as discrete, elemental units of energy calledquantaorphotons

  11. Photons • Light also behaves as a stream of particles, called photons. • Light has “wave-particle duality” , meaning that it behaves as waves and as particles. • This is a concept in quantum mechanics.

  12. (c) Black-body radiation is electromagnetic radiation that is in thermal equilibrium at a temperature T with matter that can absorb and emit without favouring any particular wavelength (d)Plank’s radiation law

  13. 6.1 Light Sources 3. Wien's Displacement Law • plot Planck's law for different temperatures • increasing temperature • more energy is emitted • the peak emission shifts toward the shorter wavelengths The temperature and the wavelength of maximum intensity satisfyTmax=constant

  14. Black-Body Radiation • Hole in a cavity is • a perfect absorber • a perfect emitter • Called a Black Body • Wien’s law

  15. Example - Wien’s Law • What is the peak radiation emitted by an object at 100oC ? • This is in the far infrared. • What T required for middle of visible range?

  16. Blackbody Radiation: Experimental Results • At 310 Kelvin (=37oC = 98.6oF), only get IR Intensity blue yellow red IR UV wavelength

  17. Blackbody Radiation:Experimental Results • At much higher temperatures, get visible • look at blue/red ratio to get temperature Intensity blue yellow red IR UV wavelength

  18. Temperature of the Sun When we look at the visible spectra of the sun, we see that it’s intensity peaks at about 500 nm (green light). From the equation:  = b/T (where b = 2.9 x 10-3m*K) we get: T = b/ = (2.9 x 10-3m*K) / 500 x 10-9m  6000 K .

  19. 4. Stefan-Boltzmann's Law 6.1 Light Sources The total energy density inside a blackbody cavity is given by integration over all wavelengths Note that Intensity increases with T Temperature must be in Kelvin, where size of one Kelvin is same as size of one degree Celsius, but T=0K is absolute zero, and T=273K = 0oC (freezing).

  20. 6.1 Light Sources 5. Klrchhoff's Law • Kirchhoff's law :an object that is a good radiator at a given wavelength is also a good absorber at the same wavelength • Stefan-Boltzmann's law for gray bodies factor : the emissivity of the surface • Recall that a good absorber is also a good emitter, and a poor absorber is a poor emitter. We use the symbol  to indicate the blackness ( =0) or the whiteness (=1) of an object.

  21. Example If you eat 2,000 calories per day, that is equivalent to about 100 joules per second or about 100 Watts - which must be emitted. Let’s see how much radiation you emit when the temperature is comfortable, say 75oF=24oC=297K, and pick a surface area, say 1.5m2, that is at a temperature of 93oF=34oC=307K: Memitted =AT4 = (5.67x10-8W/m2K4)*(.97)*(1.5m2)*(307K)4 = 733 Watts emitted!

  22. Example continued But this is not the whole story: besides emitting radiation, we receive radiation from the outside: Mabsorbed =AT4 = (5.67x10-8W/m2K4)*(.97)*(1.5m2)*(297K)4 = 642 Watts absorbed! Hence, the net power emitted by the body via radiation is: Mnet =733 Watts -642 Watts = 91 Watts.The peak of this radiation is at: peak = b/T = 2.9x10-3m*K / 307K = 9.5m which is in the infrared (as expected).

  23. 6.2 Detectors • thermal detectors based on absorption and heating If the absorbing material is black, they are independent of wavelength. • quantum detectors. based onphotoelectric effect Quantum detectors are of particular interest, both theoretical and practical; some of them are so sensitive they respond to individual quanta.

  24. 6.2 Detectors 1. Thermal Detectors slow to respond • Golay cell a thin black membrane placed over a small, gas-filled chamber. Heat absorbed by the membrane causes the gas to expand, which in turn can be measured, either optically (by a movable mirror) or electrically (by a change in capacitance). used in the infrared.

  25. 6.2 Detectors • Thermocouple a junction between two dissimilar metals. As the junction is heated, the potential difference changes. In practice, two junctions are used in series, a hot junction exposed to the radiation, and a cold junction shielded from it. The two voltages are opposite to each other; thus the detector, which without this precaution would show the absolute temperature, now measures the temperature differential. • thermopile contains several thermocouples and, therefore, is more sensitive.

  26. 6.2 Detectors • bolometer contains a metal element whose electrical resistance changes as a function of temperature; if instead of the metal a semiconductor is used, it is called a thermistor. Unlike a thermocouple, a bolometer or thermistor does not generate a voltage; they must be connected to a voltage source.

  27. 6.2 Detectors 2. Quantum Detectors • the wavelength of the light plays an important role there is a certain threshold above which there is no effect at all, no matter what the intensity • intense light and dim light cause same of an effect

  28. Photoelectric Effect Albert Einstein (1879-1955) Photoelectric effect demonstrates the particle nature of light No e- observed until light of a certain minimum E is used. Number of e- ejected does NOT depend on frequency, rather it depends on light intensity.

  29. Photoelectric Effect (2) • Classical theory said that E of ejected • electron should increase with increase • in light intensity — not observed! • Experimental observations can be explained if light consists of particles called PHOTONSof discrete energy.

  30. Discrete Packets of Energy

  31. 6.2 Detectors Light e- A V Variable power supply • plate M(photocathode) when irradiated, releases electrons (called photoelectrons) • collector plate C(anode) photoelectrons released by M are attracted by, and travel to C. As the potential V, read on an high-impedance voltmeter, is increased, the current, I, read on an ammeter, increases too, but only up to a given saturation level, because then all of the electrons emitted by M are collected by C.

  32. 6.2 Detectors if C is made negative, some photocurrent will still exist, provided the electrons ejected from M have enough kinetic energy to overcome the repulsive field at C. But as C is made more negative, a point is reached where no electrons reach C and the current drops to zero. This occurs at the stopping potential, V0. In short: A significant amount of photocurrent is present only if the collector, C, is made positive

  33. When the frequency of the light is increased, the stopping potential also increases.

  34. The electron photo-current can be stopped by a retarding potential. Increasing the light intensitydo not change the retarding potential.

  35. 6.2 Detectors • If more intense light falls on the photocathode, it will release more electrons but their energies, and their velocities, will remain the same. • The energy of the photoelectrons depends on the frequency of the light: blue light produces more energetic photo-electrons than red light. • The response of a quantum detector is all but instantaneous: there is no time lag, at least not more than 10-8 s, between the receipt of the irradiation and the resulting current.

  36. 6.2 Detectors • The light is received in the form of discrete quanta. • Part of the energy contained in a quantum is needed to make the electron escape from the surface; that part is called the work function, W. • Only the excess energy, beyond the work function, appears as kinetic energy of the electron. The maximum kinetic energy with which the electron can escape, therefore, is • KEmax = h - W • Einstein's photoelectric-effect equation.

  37. h = W + KE KE = h - W • Einstein suggested that the linear behaviour is simply a Conservation of Energy. • Energy of Light =Energy needed to get out +Kinetic Energy of electron.

  38. Example - Photoelectric Effect • Given that aluminum has a work function of 4.08 eV, what are the threshold frequency and the cutoff wavelength?

  39. 6.2 Detectors It is often convenient to measure energies on an atomic scale not in joule but in electron volt, eV. 1 eV = (1e)(1V) = 1.60 6  10-19 J

  40. Photons and Colors • Electron volts are useful size units of energy 1 eV = 1.6 x 10-19 Coul × 1V = 1.6 x 10-19 J. • radio photon: hf = 6.63 x 10-34 Js × 1 x 106 /s = 6.63 x 10-28 J = 4 x 10-15 eV • red photon: f = c/3 × 108 m/s / 7 x 10-7 m = 4.3 x 1014 Hz, red photon energy = 1.78 eV • blue: = 400 nm; photon energy = 3.11 eV .

  41. 6.2 Detectors The work function determines the longest wavelength to which a detector can respond: the lower the work function, the longer the wavelength. The lowest work functions are found among the alkali metals. Photoelectric Properties Of Some Alkali Metals Alkali Work function (eV) Threshold (nm) Sodium 2.28 543 Potassium 2.25 551 Rubidium 2.13 582 Cesium 1.94 639

  42. The Photoelectric Effect on Potassium Determine the work function W KE=(hc)(1/) － W

  43. From the graph: The plot is essentially KE vs 1/, so that since KE=hc/－W The intercept when (1/)=0 give W=－KE=－(－2eV)=2eV To obtain Planck’s constant h, we need the slope S Then h=S/c. S=(4－(－2))/(5－0) × 10-3=1.2 * 103 eV nm h = 1.2 × 103 × 1.602 × 10-19×10-9 /(3 × 108)J s = 6.4× 10-34 J s cf (6.626 × 10-34 J s)

  44. 6-3. Practical Quantum Detectors In contrast to thermal detectors, quantum detectors respond to the number of quanta, rather than to the energy contained in them.

  45. - + - hv - e- 6.3 Practical Quantum Detectors • The simplest type is probably the vacuum phototube, an example of a photoemissive detector. • Light strikes photocathode (-) • Photocathode emits photoelectrons • Photoelectrons accelerate toward anode (+) • flow of electrons = current • current proportional to # photons incident on photocathode

  46. quantum efficiency:the ratio of the number of photoelectrons released to the number of photons received. • Ordinarily, this efficiency is no higher than a few percent. • Several diodes are combined in series to form a photomultiplier, the efficiency becomes much higher. • Light strikes photocathode (-) • Photocathode emits photoelectrons • Photoelectrons accelerate toward series of increasingly positive anodes (+) at which photoelectrons and secondary electrons are emitted (dynodes) • Electrons accelerated toward collection anode

  47. 6.3 Practical Quantum Detectors • A photocell is the solid-state equivalent of the vacuum photodiode; most often it is a semiconductor. • A semiconductor conducts electricity better than an insulator but not as well as a conductor. • In an insulator, the electrons are tightly bound to their respective atoms. • In a metal, the electrons can move freely; hence, even a small voltage applied to the conductor will cause a current.

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