Light and Matter. Slide 1 / 62. Slide 2 / 62. Slide 3 / 62. Light and Sound. New Jersey Center for Teaching and Learning

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1 New Jersey Center for Teaching and Learning Slide 1 / 62 Progressive Science Initiative This material is made freely available at and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others. Click to go to website: Slide 2 / 62 Light and Matter Light and Sound Slide 3 / 62 In 1905 Einstein derived an equation relating mass and energy. You should be familiar with this equation: E = mc 2 This equation has been changed a bit since, but a relationship has now, for the first time in history, been established between matter and energy, and between physics and chemistry.

2 Slide 4 / 62 Light and Sound Because Einstein was able to prove a relationship between matter and energy, we today can understand more about matter by learning all about energy. We can see this relationship between energy and matter specifically when we look at some of the unusual properties of the wave nature of energy. The Nature of Light: Wave or Particle? Slide 5 / 62 The nature of light has been debated for thousands of years. In the 1600's, Newton argued that light was a stream of particles. Huygens countered that it was a wave. Both had good arguments, but neither could prove their case. particle! wave! Young's Double Slit Experiment Slide 6 / 62 In 1801, Thomas Young settled the argument (apparently) with his Double Slit Experiment. Later, when we look at the results of Young's experiment we will see one of the unusual properties of energy that we were talking about. But first, we must understand waves. To study the properties of waves we can look at any type of wave, from the waves in a body of water, to the sound waves produced by speakers. Waves are waves. Click here to see a Veritasium video on Young's original Double Slit Experiment

3 Young's Double Slit Experiment Slide 7 / 62 Young tested to see if light was a wave by seeing if it created an interference pattern when it went through two slits, like a wave would. slit screen measurement screen light source d L Young's Double Slit Experiment Slide 8 / 62 This photo is of light (of one color) striking a distant screen after passing through 2 slits. This only makes sense if light is a wave. slit screen measurement screen d x light source L Diffraction and Interference Slide 9 / 62 The double slit experiment relies on two properties of waves: diffraction and interference Each slit generates a new wave due to diffraction. Those waves then either constructively or destructively interfere on a far away screen. S1 S 2 viewing screen

4 1 What principle is responsible for light spreading as it passes through a narrow slit? Slide 10 / 62 A B C D diffraction polarization dispersion interference Double-Slit Maxima and Minima Slide 11 / 62 Interference occurs because each point on the screen is not the same distance from both slits. Depending on the path length difference, the wave can interfere constructively (bright spot) or destructively (dark spot). Double-Slit Maxima and Minima Slide 12 / 62 The bright lines that appear on the screen are called maxima. The dark lines are called minima. Maxima are evenly spaced, and a minima occurs between each pair of maxima.

5 2What principle is responsible for alternating light and dark bands when light passes through two or more narrow slits? Slide 13 / 62 A B C D diffraction polarization dispersion interference If Light is a Wave What exactly is waving? Slide 14 / 62 In sound waves, we know it's the pressure in the air. In any simple harmonic motion there has to be two forms (or levels) of energy and a means to move between them. But what does that mean for light? Accelerating Charges create E-M waves Slide 15 / 62 A great way to start this up is to make a charge (like an electron) accelerate. That creates a changing electric field... which creates a changing magnetic field... which creates a changing electric field... which creates a changing magnetic field... which creates a changing electric field... which creates a changing magnetic field... Electromagnetic Wave Direction

6 James Maxwell Slide 16 / 62 In Scotland in the late 1800's, James Maxwell, combined together the known equations of electricity and magnetism, and added one, to create Maxwell's Equations. Maxwell's Equations Slide 17 / 62 Maxwell's Equations Gauss's Law Gauss's Law for Magnetism Faraday's Law of Induction Ampere's Law Speed of Light Slide 18 / 62 He found they predicted that energy could move between two forms (electric and magnetic) and that disturbance must travel through space at a speed of 3.0 x 10 8 m/s. This very much agreed with the known speed of light. 3.0 x 10 8 m/s is the speed of light in a vacuum.

7 Creating Electromagnetic Waves Slide 19 / 62 In physics we learned that a changing magnetic field produces an electric field. Maxwell showed that a changing electric field produces a magnetic field as well. Once these changing fields are first started up, they keep creating each other...and travel on their own. These traveling fields are called electromagnetic waves. Electromagnetic Wave Direction 3 An electric field is produced by a Slide 20 / 62 A constant magnetic field. B changing magnetic field. either a constant or a changing C magnetic field. D gravitation 4 A changing electric field will produce a Slide 21 / 62 A current. B gravitational field. C magnetic field. D a gravitational field

8 Slide 22 / 62 c= Light is an Electromagnetic Wave Young showed that light is a wave. Maxwell showed that electromagnetic waves exist and travel at the speed of light. Light was shown to be an electromagnetic wave. The frequency of an electromagnetic wave is related to its wavelength. For electromagnetic waves (including light), in a vacuum: c = speed of light λ = wavelength (m) = frequency (Hz or s -1 ) c = λ The Electromagnetic Spectrum Slide 23 / 62 c = λ All electromagnetic radiation travels at the same velocity: the speed of light (c) c = 3.00 x 10 8 m/s. 5 All electromagnetic waves travel through a vacuum at Slide 24 / 62 A B C D the same speed. speeds that are proportional to their frequency. speeds that are inversely proportional to their frequency. speeds too slow to measure

9 6 In a vacuum, the velocity of all electromagnetic waves: Slide 25 / 62 A is zero. B is m/s. C depends on the frequency. D depends on their amplitude. 7 For a wave, the frequency times the wavelength is the wave's. Slide 26 / 62 A speed. B amplitude. C intensity. D power. 8 The wavelength of light that has a frequency of 1.20 x Hz is. Slide 27 / 62 A 25 m B 2.5 x 10-5 m C m D 2.5 m c = λv c = 3.00 x 10 8 m/s

10 9 Electromagnetic radiation travels through a vacuum at a speed of. Slide 28 / 62 A 186,000 m/s B 125 m/s C 3.00 x 10 8 m/s D It depends on wavelength 10 What is the frequency of red light whose wavelength is 600 nm? Slide 29 / 62 A 5.0 x Hz B 1.0 x Hz C 1.5 x Hz D 2.0 x Hz c = λv c = 3.00 x 10 8 m/s 11 Plants absorb red light with a frequency of 5 x Hz while reflecting green light with a frequency of 5.5 x Hz. What must be true of green light compared to red light? Slide 30 / 62 A B Green light has a longer wavelength than red light Green light has a shorter wavelength than red light C D Green light travels at a slower speed than red light Green light travels at a faster speed than red light E Green and red light have the same wavelength

11 Blackbody Radiation Slide 31 / 62 All objects emit electromagnetic radiation which depends on their temperature: thermal radiation. A blackbody absorbs all electromagnetic radiation (light) that falls on it. Because no light is reflected or transmitted, the object appears black when it is cold. However, black bodies emit a temperaturedependent spectrum termed blackbody radiation. For example, the temperature of the above Pāhoehoe lava flow can be estimated by observing its color. click here for a PHET simulation of the blackbody spectrum Blackbody Radiation Slide 32 / 62 This figure shows blackbody radiation curves for three different temperatures. As can be seen the frequency and intensity changes depending on the temperature of the substance. Classical physics couldn't explain the shape of these spectra. Planck s Quantum Hypothesis Slide 33 / 62 The wave nature of light could not explain the way an object glows depending on its temperature: its spectrum. In 1900, Max Planck explained it by assuming that the atoms that make up the objects only emit radiation in quantum amounts. These days, this assumption is regarded as the birth of quantum physics and the greatest intellectual accomplishment of Planck's career. Quantum: discrete quantity of electromagnetic radiation

12 Planck s Postulate Slide 34 / 62 Energy and frequency are directly related E = hv where h is Planck s constant (6.63 x J*s) and v is the frequency of the light Planck s Quantum Hypothesis Slide 35 / 62 According to Planck's hypothesis, since only certain frequencies of light were emitted at varying temperatures, the amount of energy put into a substance triggered that substance to release a very specific type of light. In other words, if we think of this like a person walking up a flight of stairs, the person cannot reach a certain height unless first raising his or her legs to the height of the specific steps. Planck s Quantum Hypothesis Slide 36 / 62 Planck didn't believe this was real...it just worked. It was like working from the answers in the book you see that it works, but you have no idea why. Atoms only having steps of energy? This didn't make sense. Why couldn't they have any energy? Planck thought a "real" solution would eventually be found...but this one worked for some reason. Which brings us to our next mystery...

13 The Photoelectric Effect Slide 37 / 62 When light strikes a metal, electrons sometimes fly off causing an electric current. Classical physics couldn't explain some specific features about how the effect works. So Einstein used Planck's idea to solve it. The Photon Slide 38 / 62 If atoms can only emit light in packets of specific sizes, maybe light itself travels as packets of energy given by Planck's formula. evacuated chamber E = hv Radiant energy metal surface where h is Planck s constant (6.63 x J*s) e- voltage source Current indicator He called these tiny packets of energy or light photons. Particle Theory of Light Slide 39 / 62 This particle theory of light assumes that an electron absorbs a single photon and made specific predictions that proved true. For instance, the kinetic energy of escaping electrons vs. frequency of light shown below: KEmax of electrons Frequency of light (v) This shows clear agreement with the photon theory, and not with wave theory. This supports the proposition that light is made of particles (photons) and therefore light is not a wave.

14 Wave-Particle Duality Slide 40 / 62 Earlier we proved that light is a wave. Now we've proven that light is a particle. So which is it? Wave-Particle Duality Slide 41 / 62 Particle? Wave? This question has no answer; we must accept the dual wave-particle nature of light. While we cannot imagine something that is both a wave and a particle at the same time; that turns out to be the case for light. Check out this animation about the Wave-Particle Duality Like that? Here's one more to watch 12 The ratio of energy to frequency for a given photon gives Slide 42 / 62 A its amplitude. B its velocity. C Planck's constant. D its work function. E = hv c = λv h = 6.63 x 10 J-s -34 c = 3.00 x 10 m/s 8

15 13 What is a photon? Slide 43 / 62 A an electron in an excited state B C D a small packet of electromagnetic energy that has particle-like properties one form of a nucleon, one of the particles that makes up the nucleus an electron that has been made electrically neutral 14 The energy of a photon depends on Slide 44 / 62 A its amplitude. B its velocity. C its frequency. D none of the given answers 15 The photoelectric effect can be explained assuming Slide 45 / 62 A that light has a wave nature. B that light has a particle nature. C that light has a wave nature and a particle nature. D none of the above

16 16 The energy of a photon that has a frequency 110 GHz is Slide 46 / 62 A J -20 B J C J D J E = hv c = λv h = 6.63 x 10 J-s -34 c = 3.00 x 10 m/s 8 17 The frequency of a photon that has an energy of 3.7 x J is Slide 47 / 62 A Hz B Hz C J D J E J E = hv c = λv h = 6.63 x 10 J-s -34 c = 3.00 x 10 m/s 8 18 The energy of a photon that has a wavelength of 12.3 nm is Slide 48 / 62 A J B J C J D J E J E = hv c = λv h = 6.63 x 10 J-s -34 c = 3.00 x 10 m/s 8

17 19 If the wavelength of a photon is halved, by what factor does its energy change? Slide 49 / 62 A 4 B 2 C 1/4 D 1/2 E = hv c = λv h = 6.63 x 10 J-s -34 c = 3.00 x 10 m/s 8 20 Compared to UV light with a wavelength of 300 nm, red light has half the energy. What must be the wavelength of this red light? Slide 50 / 62 A 150 nm B 300 nm C 600 nm D 900 nm E 450 nm Slide 51 / 62 Energy, Mass, and Momentum of a Photon Clearly, a photon must travel at the speed of light, (since it is light) Special Relativity tells us two things from this: The mass of a photon is zero. The momentum of a photon depends on its wavelength.

18 Energy, Mass, and Momentum of a Photon Slide 52 / 62 m = 0 p = hv c p = h λ and since c = λv This last equation turned out to have huge implications. Matter as a wave? Slide 53 / 62 Taking all of this into account, in 1924, French physicist Louis de Broglie asked: "If light can behave like a wave or a particle, can matter also behave like a wave?" He found that amazingly, it does! Wavelength of Matter Slide 54 / 62 de Broglie combined p = h/ λ with p = mv to get The wavelength of matter l = h λ = mv in other words WAVE = PARTICLE This wavelength is really small for normal objects, so it had never been noticed before. But it has a dramatic impact on the structure of atoms.

19 Slide 55 / 62 Wave Nature of Matter The de-broglie hypothesis that particles have wave-like properties needed to be supported by experiment. In fact, in a Nobel prize winning experiment, Davisson and Germer of Bell Labs found that electrons could be diffracted (remember the two slit experiment) just like waves. Electron wavelengths are often about m, about the size of an atom, so the wave character of electrons is important. Wave Nature of Matter Slide 56 / 62 Electrons fired one at a time towards two slits show the same interference pattern when they land on a distant screen. The "electron wave" must go through both slits at the same time...which is something we can't imagine a single particle doing...but it does. Click here for a video with more explanation of all this! The most amazing experiment ever! Slide 57 / 62 These photos show electrons being fired one at a time through two slits. Each exposure was made after a slightly longer time. The same pattern emerges as was found by light. Each individual electron must behave like a wave and pass through both slits. But each electron must be a particle when it strikes the film, or it wouldn't make one dot on the film, it would be spread out. This one picture shows that matter acts like both a wave and a particle.

20 21 What is the wavelength of a 0.25 kg ball traveling at 20 m/s? Slide 58 / 62 l = h = mv h = 6.63 x J-s 22 What is the wavelength of a 80 kg person running 4.0 m/s? Slide 59 / 62 l = h = mv h = 6.63 x J-s 23 What is the wavelength of the matter wave associated with an electron (m e = 9.1 x kg) moving with a speed of m/s? Slide 60 / 62 l = h = mv h = 6.63 x J-s

21 24 What is the wavelength of the matter wave associated with an electron (m e = 9.1 x kg) moving with a speed of m/s? Slide 61 / 62 l = h = mv h = 6.63 x J-s Slide 62 / 62 Why does all this "Matter"? "Are not the gross bodies and light convertible into one another, and may not bodies receive much of their activity from the particles of light which enter their composition?" - Newton Since matter and energy are now understood to share certain properties (wavelength for example) the interaction of matter with light has allowed us to probe the nature of matter itself, from the structure of the atom to the unique behavior of molecules. The structure and behavior of matter is the domain of the chemist!