Light & Matter I

1. Light & Matter I

2. Here!  Attendance Quiz Are you here today? (a) yes (b) no (c) I hear there is another team in SoCal besides the Angels

3. Exam #1 • The first midterm exam is next Monday, 4/18, during the first half of class - please come on time (or better yet, early) • It will be multiple choice, and you will take it using a 50-question scantron, so make sure to bring one to class! • It will cover the material in Chapters 1, 3, 4, 5 of the optional textbook • Most questions will look something like the in-class “Conceptual Questions” or the Lecture Tutorial and Ranking Task questions - your LT book is your textbook to study from • Cell phones must be off and put away during the exam (as they should be in every class). If I see or hear a cell phone that is grounds to fail the exam! • We will be studying Special Relativity that day as well, so if you have a book, you will want to do the reading as usual

4. Today’s Topics • What is Light? • Waves v. particles • Electromagnetic Spectrum • Wavelength, frequency, energy • Properties of Matter • Atoms and the Periodic Table • Atomic structure • Interaction of Light with Matter • Emission and absorption of photons by electrons • Types of Spectra • Continuous • Emission • Absorption

5. What is Light? • Light is a form of energy (warmth of the Sun) • Energy is measured in Joules or electron volts (eV) 1 eV = 1.6 x 10-19 J • Power measured energy per unit time; 1 Watt (W) = 1 J/sec • Light carries information (e.g., color, brightness)

6. Interaction of Light with Matter (macro) • Light and matter interact on a macroscopic level in four ways • Emission • Absorption • Transmission • Reflection/scattering • The picture below illustrates all four: which objects emit, which absorb, which transmit, and which reflect or scatter?

7. What is Light? Yes! • Is light a wave or a particle? • As the wave approaches the bird, the buoy bobs up-and-down • The number of times/second is called the frequency (f ) in Hz • The time between crests (or troughs) is the period (T = 1/f) • The spacing between the crests (or troughs) is the wavelength () • The speed of the wave is 1 wavelength per period (v = /T = f) Wave active figure

8. f  What is Light? • Light is electromagnetic (EM) waves • What is waving? Electric and magnetic fields. • c = 300,000 km/s for all EM radiation • Since c = f   = constant for all EM waves, as frequency (f) increases, wavelength () decreases, and vice versa

9. f E  What is Light? • A baseball is a particle; it is a discrete object • Waves are not generally considered to be discrete - by definition they are spread out • However, light does come in packets called photons • Each photon carries an energy proportional to its frequency

10. f, E  c The Electromagnetic Spectrum • A prism shows visible light spread out into colors (ROY G. BIV) Demo • Herschel showed (c. 1800) that there is energy outside the visible spectrum, by placing a thermometer beyond the red (infrared radiation) • EM radiation exists at all possible wavelengths and frequencies • Visible radiation runs from 400-700 nm, blue to red (1 nanometer = 10-9 m) Radio (Optical)

11. f, E  c The Electromagnetic Spectrum • EM spectrum song (Real Player)

12. Light Quiz I Blue light is (compared to red light), • shorter wavelength • longer wavelength • higher energy photons • (a) and (c) • none of the above

13. Light Quiz II Which travels fastest? • X rays • Ultraviolet • Visible • Radio waves • They all travel at the same speed

14. Lecture Tutorial: Electromagnetic Spectrum, pp. 45-47 • Work with one or more partners - not alone! • Get right to work - you have 10 minutes • Read the instructions and questions carefully. • Discuss the concepts and your answers with one another. Take time to understand it now!!!! • Come to a consensus answer you all agree on. • Write clear explanations for your answers. • If you get stuck or are not sure of your answer, ask another group. • If you get really stuck or don’t understand what the Lecture Tutorial is asking, ask me for help.

15. Light Quiz III Put the following kinds of light in order, according to wavelength, short to long: • Visible light, ultraviolet (UV), infrared (IR), radio, microwaves, X rays, gamma rays • Gamma rays, UV, visible, X-rays, IR, radio • Gamma rays, X rays, UV, visible, IR, radio • X rays, UV, visible, IR, radio, gamma rays • UV, visible light, IR, gamma rays, X rays

16. Properties of Matter • Matter comes in a number of different types (zoo of particles) • However, the majority of the matter we can detect directly is made up of atoms • There are 92 naturally occuring elements, and the pure form of each element is made of atoms • Combinations of atoms form molecules • All atoms consist of 3 kinds of particles: electrons, protons, and neutrons

17. Structure of the Atom • Rutherford showed, in 1909, that most of the mass of an atom is in a very small nucleus, surrounded by electrons • The nucleus contains both protons, with electric charge +1, and neutrons with electric charge 0, while the electrons carry charge 1 • The mass of the proton and neutron are very similar and both are about 2000 times more massive than the electron • Atoms with equal numbers of protons and electrons are neutral • The atomic number (Z) is equal to the number of protons • The atomic mass number (A) is the number of protons & neutrons

18. Structure of the Atom • Since unlike electric charges attract, the electrons are bound to the atom by their electrical attraction for the positive protons • However, like charges repel, so why don’t nuclei (heavier than H) fly apart due to the electrical repulsion of the protons, which are very close together? • The answer is the strong nuclear force, which binds together nucleons (protons and neutrons) when they are closer than ~1015 m • Thus, rnucleus ~ 1015 m, compared to the typical size of an atom of ~ 1010 m • If your fist were a nucleus, the atom would be about 10 km across! • The nucleus is 100,000 smaller in diameter, and 1,000,000,000,000,000 times smaller in volume, but contains >99.9% of the mass!

19. Isotopes • The number of protons (Z) determine the type of element • For a given element, the number of neutrons can vary • These different versions of the same element are called isotopes • For example normal hydrogen (1H) has a nucleus of 1 proton • Deuterium (2H) has 1 proton and 1 neutron • Tritium (3H) has 1 proton and 2 neutrons • Carbon also has isotopes (e.g., 12C, 13C, 14C) • 14C is radioactive (not stable) and is used in “carbon dating”

20. Isotope Quiz How is the isotope 238U different from 235U? • It has more protons • It has more neutrons • It has more electrons • All of the above • None of the above

21. Structure of the Atom • In studying orbits, we found that planets further from the Sun had more gravitational potential energy, but less kinetic energy (they moved slower) • It turns out that their total energy is greater, the further they are from the Sun • In a similar way, an electron far from the nucleus has more total energy than one closer • However, unlike a planet, an electron in an atom cannot have just any energy - quantum mechanics dictates that only certain energy levels are allowed • The picture at right is a schematic of an atom, showing 5 of the allowed energy levels of the electrons in the atom

22. nucleus ground state; n = 1 first excited state; n = 2 Structure of the Atom • The grey dot in the center represents the nucleus, which would be far too small to see in such a diagram • The circles represent the allowed energy levels of the atom (indicated by the letter n) - only 5 are shown • The closest circle to the nucleus represents the lowest possible energy level for an electron in the atom - and is called the ground state of the atom (n = 1) • The next closest circle represents the next highest energy level, and is called the first excited state of the electron (n = 2) • The spacing between the lines is supposed to represent the difference in energy between these states • Note that the gap between the ground state and first excited state is the largest, and the gaps between states becomes progressively smaller moving to higher and higher energy states • As n, the energy levels approach a finite value; the energy difference between this value and the ground state is called the ionization energy of the atom

23. Interaction of Light with Matter (micro) • When a photon of light encounters an atom, if the energy of the photon exactly matches the energy gap between the energy level the electron is in and a higher energy level, the the photon can be absorbed, and the electron will “jump” from the lower to the higher level • For example, in Hydrogen, the gap between the ground state and first excited state is 10.2 eV • Thus, if a photon of energy 10.2 eV (corresponding to a wavelength of 121.6 nm, or UV light) comes along, and there is an electron in the ground state, then the atom can absorb the photon, and the electron in the ground state will move to the first excited state e e

24. Interaction of Light with Matter (micro) • If the electron then encountered another photon with an energy equal to the gap between the first and second excited states, it could absorb that photon and move up again • In Hydrogen, the gap between the first and second excited state is 1.9 eV • Thus, if the second photon had an energy of 1.9 eV (corresponding to a wavelength of 656.3 nm, or red light) then the atom can absorb this photon as well, and the electron will move to the second excited state e e

25. Interaction of Light with Matter (micro) • Alternatively, the electron in the first excited state may then return to the ground state and emit a photon of the same energy as the original photon, 10.2 eV • Thus, the wavelengths of emission and absorption are the same for any given atom • Each atom has its own unique energy levels, determined by the charge of the nucleus, and the interactions of the nucleus with the electrons and of the electrons with each other • Thus, each element has a unique spectrum, sometimes called a spectral fingerprint, meaning it can only absorb and emit specific wavelengths of light • This uniqueness can help identify an element by its spectrum e e

26. Interaction of Light with Matter (micro) • If the energy of the photon is greater than the ionization energy of the atom (the difference between the ground state and the limit of the energy levels as n, then the electron can absorb the photon and escape from the atom • The atom is then ionized, and it has an electric charge of +1 • In Hydrogen, the ionization energy is 13.6 eV • Thus, if a photon of energy ≥13.6 eV (corresponding to a wavelength of 91.2 nm, or UV light) comes along, an electron in the ground statecan absorb that photon, and the electron escapes, leaving an ionized atom (in this case a bare proton) e

27. Active Figure - Ch. 5 The Three Types of Spectra: Continuous, Emission and Absorption

28. Interaction of Light with Matter (micro) Things to remember as you do your activity on Light and Atoms • Atoms absorb light when they move from lower to higher energy levels • Atoms emit light when they move from higher to lower energy levels • In order for a photon to be absorbed by an atom, the energy of the photon must be exactly the same as the difference in energy between an energy level occupied by an electron and a higher energy level • The spacing between the levels gets smaller as the energy levels get higher • The absorption and emission spectra of an element are identical - providing a unique “fingerprint” e e

29. Lecture Tutorial: Light and Atoms, pp. 63-67 • Work with one or more partners - not alone! • Get right to work - you have 15 minutes • Read the instructions and questions carefully. • Discuss the concepts and your answers with one another. Take time to understand it now!!!! • Come to a consensus answer you all agree on. • Write clear explanations for your answers. • If you get stuck or are not sure of your answer, ask another group. • If you get really stuck or don’t understand what the Lecture Tutorial is asking, ask me for help.