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Fig. 1. Photograph of some of the equipment used in the optics experiments. INSTRUMENT CALIBRATION The Ocean Optics HR2000+ Fiber Optic Spectrometer measures a light intensity distribution density (light intensity per unit band of wavelength, with units microwatts per square centimeter per nanometer, µW/cm 2 -nm) as a function of wavelength. Both the wavelength axis and the light intensity distribution density axis must be calibrated. An Ocean Optics HG-1 Mercury Argon Calibration Light Source is used for for calibrating the wavelength, and an Ocean Optics LS-1 Calibration Light Source is used for calibrating the light intensity distribution density. After using these calibration sources, be sure to turn them off, because the bulbs have a finite lifetime. THEORY The Photon The photon is the most abused concept in all of science. The truth of the matter may be seen in a quote from Albert Einstein: “All these fifty years of conscious brooding have brought me no nearer to the answer to the question, ‘What are light quanta?’ Nowadays every Tom, Dick and Harry thinks he knows it, but he is mistaken.” (1954) Virtually all “modern physics” textbooks and “photon” web sites claim that Einstein and the photoelectric effect (and other experiments) showed the particle nature of light. However, the “particle nature of light” and the “photon” were thought up by others after Einstein’s 2
explanation of the photoelectric effect, and Einstein did not care for these concepts. The truth is that there is absolutely no experiment which requires that light behave like a particle; it is completely sufficient that charges (including those in “detectors”) behave according to quantum mechanics. There is the greatly reduced concept that a photon is related to the quantization of the electromagnetic field. It is a greatly reduced concept because in all of physics there are only a few phenomena which actually require the quantization of the electromagnetic field (which remains absolutely not particle-like): 1) spontaneous emission from atoms, 2) the Lamb shift, 3) the anomolous magnetic moment of the electron, and a few other more obscure effects. Virtually all experiments in optics can be explained completely with classical electromagnetic fields, and there is no need for a “photon”. The photon can be a useful concept in perturbation approximations (as in Feynman diagrams), but Nature does not have any need for perturbation approximations. If you want to be a practitioner of rigorous physics, don’t be like Tom, Dick and Harry; if you simply stop using the term photon, or in very rare cases refer to “the quantized electromagnetic field”, you will be right every time. [?] Blackbody Radiation Blackbody radiation [?,?] is electromagnetic radiation from a particular type of source. In the laboratory, blackbody radiation is approximated by the radiation from a small hole entrance to a large cavity. Any radiation entering the hole would reflect off the walls of the cavity multiple times and would be unlikely to again find the entrance hole and escape; such a perfect absorbance of light would make the cavity and hole appear to be “black”. Arguments invoking thermal equilibrium show that the ideal absorbance is independent of wavelength and shape of the cavity and depends only on the temperature of the cavity walls, and also shows that the blackbody will emit the same amount of radiation as it absorbs. The escaping radiation is characterized by an intensity distribution density as a function of wavelength λ (and parameterized by the temperature T): ε e (λ; T). Because this quantity is a function of wavelength, it is often referred to as a “spectral” density, or sometimes simply “spectrum”. Its units are intensity per band of wavelength, conveniently expressed as microwatts per square centimeter per nanometer, or µW/cm 2 -nm. It is a “density” in that it must be multiplied by a wavelength bandwidth in order to obtain an actual radiation intensity (in µW/cm 2 ); thus ε e (λ; T)dλ gives the intensity of the radiation having wavelengths between λ and λ + dλ. In 1901 Max Planck showed that for an ideal blackbody ε e (λ; T) = 2πhc2 λ 5 1 e hc/λk BT − 1 (1) where h = 6.626×10 −34 J-s is Plank’s constant, c = 3.00×10 8 m/s is the speed of light, and k B = 1.38×10 −23 J/K is Boltzmann’s constant. Plots of ε e (λ; T) versus λ for several values of T are shown in Fig. 2. 3
Page 1: OPTICS INTRODUCTION The optics lab
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