Quantum Aspects of Light and Matter Notes on Quantum Mechanics
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Quantum aspects of light and matter Notes on Quantum Mechanics http://quantum.bu.edu/notes/QuantumMechanics/QuantumAspectsOfLightAndMatter.pdf Last updated Tuesday, September 20, 2005 8:07:07-05:00 Copyright © 2004 Dan Dill ([email protected]) Department of Chemistry, Boston University, Boston MA 02215 Near the end of his life Albert Einstein wrote, “All the fifty years of conscious brooding have brought me no closer to the answer to the question: What are light quanta? Of course today every rascal thinks he knows the answer, but he is deluding himself.” We are today in the same state of “learned ignorance” with respect to light as was Einstein. Arthur Zajonc, "Light Reconsidered," Optics & Photonic News, October 2003 So, chemistry is about atoms, but atoms are too small to "see" directly. The reason is that our eyes are sensitive to the visible range of light, corresponding to wavelengths from 380 nm (deep violet) to 740 nm (dark red), but even the shortest of these wavelengths is much, much larger than atoms, which are about 1 Þ = 10-8 cm = 0.1 nm across. What to do? Our only option is to explore the structure of atoms indirectly, using a fortunate feature of atoms that they do absorb and emit light at precisely defined wavelengths. It turns out that atoms of each element in the periodic table have a characteristic pattern of interaction with light, and this pattern, called a spectrum, serves as a fingerprint to identify the atoms of the different elements. The simplest example is the spectrum of atomic hydrogen. Here is the absorption spectrum of hydrogen atoms in the visible wavelength range. Absorption spectrum of gaseous hydrogen atoms at high temperature. Wavelength increases from 380 nm on the left to 740 nm on the right The dark lines mark those wavelengths of the light that hydrogen atoms absorb. In the visible region of the spectrum there are only four such absorption lines for hydrogen atoms. Other atoms typically absorb light at many, many more different wavelengths. Absorption spectra are typically seen in light emitted from stars as it passes through clouds of atoms. In the laboratory it is more typical to see emission spectra. Here is a representation of the emission spectrum of hydrogen atoms in the visible wavelength range. Emission spectrum of gaseous hydrogen atoms at high temperature. Wavelength increases from 380 nm on the left to 740 nm on the right A nice display of the emission spectrum of the Sun and the atomic absorption spectra of hydrogen, helium, mercury and uranium is at http://quantum.bu.edu/images/atomSpectra.jpeg. To keep things as simple as possible, let's set as our goal trying to understand the hydrogen spectrum and what this understanding can teach us about the atom. 2 Quantum aspects of light and matter à Properties of waves The very first step in using the interaction of light with atoms to sort out what is going on inside atoms is to understand what light itself is. In simplest terms, light is paired electric and magnetic field that oscillate in unison perpendicular to the direction of travel of the light. The magnetic field has a negligible effect on atoms compared to the electric field and so we'll focus our attention on the oscillations of the electrical component of light. (Because of the simultaneous presence of oscillating electric and magnetic fields, light is also called electromagnetic radiation. Sometime, too, the term light is used to mean just the visible range of light. We will use light to mean electromagnetic radiation, independently of whether it is visible to our eyes.) To do this, we begin by describing the mathematics of waves. A wave is the oscillatory variation of some property in time and in space. If we focus on a particular point in space, the value of the changing property—the height of the wave—is seen to oscillate about some average value. If instead we focus on a particular part of the wave, such a one of its crests—a point of maximum amplitude—the wave is seen to move through space. Examples of waves are Wave type Changing quantity water height of water sound density of air chemical concentration light electric and magnetic fields A review of sines and cosines: SOH-CAH-TOA The trigonometric functions sine, cosine, and tangent are used in working with waves. Hopefully these are familiar to you from high school, but here is a brief review of these functions and their values. The sine of the angle q (the Greek letter "theta"), sin q , subtended by a right triangle with hypotenuse H and side O opposite the angle q is O H. The cosine of the angle q, cos(q), subtended by a right triangle with hypotenuse H and side A adjacent to the angle q is A H. The tangent of the angle q, tan(q), subtended by a right triangle side O oppositeH L the angle q and side A adjacent to the angle q is O A. ê ê Show that a consequence of these definitions is that the tangent if an angle is the ratio of itsê sine and cosine, tan q = sin q cos q . The so-called "American Indian chief" mnemonic SOH-CAH-TOA incorporates these definitions: SOH means Sine is Opposite overH Hypotenuse;L H L ê CAHH L means Cosine is Adjacent over Hypotenuse; and TOA means Tangent is Opposite over Adjacent. For angles greater than 90 °, the following identities can be used to express the values of the trigonometric functions in terms of those for angles in the range 0 §q§90 °. sin a ≤ b = sin a cos b ≤ cos a sin b cos a ≤ b = cos a cos b ¡ sin a sin b H L H L H L H L H L H L H L H L H L H L Copyright © 2004 Dan Dill ([email protected]). All rights reserved Quantum aspects of light and matter 3 For example, at 45 ° the sides and hypotenuse of a right triangle are in the ratio 1 2 = 0.71. This means sin 45 ° = cos 45 ° = 1 2 . Using the identities, we can evaluate the cosine of 135° to be è!!! cos 135 ° ë è!!! H L = Hcos 90L ° +ë45 ° = cos 90 ° cos 45 ° - sin 90 ° sin 45 ° H =L 0 μ 1 2 - 1 μ 1 2 H L =-1 2 H L H L H L H L è!!! è!!! ë ë Use the identities to show that sin -45 ° =-1 2. è!!! ë The two trigonometric identities are so useful that it is well worthwhile to memorize them. è!!! H L ë Usually it is easiest to determine the values of sine and cosine of an angle with reference to plots of the sine and cosine and a few key values. Here are the plots, with key values marked. sin q , cos q 1 H L H L 0.5 q p p 3 p 2p ÅÅÅÅÅÅÅÅÅÅ ÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ 2 2 -0.5 -1 Plot of the sine (black curve and points) and cosine (gray curve and points). The points on the curve are for angles 0, p 6 = 30 °, p 4 = 45 °, p 3 = 60 °, p 2 = 90 °, etc. Here is a table of the values of sine and cosine for key angles q. ê ê ê ê q degrees q radians sin q cos q 0001 30 p 612 3 2 H L H L H L H L 45 p 4121 2 è!!! 60 p ê 3 3ê 21ë2 è!!! è!!! 90 p ê 210ë ë è!!! Values of sine and cosine for key anglesê q. Key decimalë valuesê are 0, 0.5, 1 2 = 0.71, 3 2 = 0.87, and 1. ê The decimal values for the key angles are 0, 0.5, 1 2 = 0.71,è!!!! 3 è2!!!!= 0.87, and 1. It is easy to ë ë memorize the values of sine and cosine—which you should do—since their values in the first quadrant, 0 §q§p 2, determine the values in the other three quadrants in way that is easy to see è!!! è!!! using a sketch of the two functions. Once these valuesë are memorized,ë determining the value of the sine or cosine of an angle is not hard at all. ê Copyright © 2004 Dan Dill ([email protected]). All rights reserved 4 Quantum aspects of light and matter What is sin 405 ° ? Answer: 1 2 = 0.71. What is tan -45 ° ? Answer: -1. è!!! H L ë What is sin -30 ° ? Answer: -1 2. H L What is cos -180 ° ? Answer: -1. H L ê Representing waveH propertiesL mathematically Waves can be expressed mathematically in terms of sine or cosine curves. The value of the curve at a particular time and location represents the amplitude of the wave at that time and location. For example, to represent a sound wave with a sine curve, the value of the sine curve at a given time at each point in space corresponds to the pressure, relative to ambient pressure, at that time and location. What does a sine curve represent for a water wave? What does a sine curve represent for the electric field part of a light (electromagnetic) wave? The most striking feature of waves is that they move through space (standing waves, which oscillate in place, are a special case). To keep things as simple as possible, let's assume we have a wave only along one direction, which we call x. If we look at a wave at a particular instant in time, it is a sine curve, say.