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Interested in exploring science or math teaching as a career? Start with Step 1: EDUC 2020 (1 credit) Real experience teaching real kids! No commitment to continue with education courses Registration priority for math, science, and open option majors Financial benefits such as scholarships & internships (if you join the Teacher Certification program) Want more info? google: CU Teach Or visit our website at: www.colorado.edu/cuteach 1 What s going to be on the third midterm? Ch. 12, wave optics Ch. 13, scattering and polarization Ch. 14, holography Ch. 8, binocular vision & depth perception Ch. 15.3 15.4, modern physics Tuesday, May 4, 4:30 6:00 p.m. (90 mins. only) in our classroom. Exam format: same as the first two exams. 2

Lec. 29, Tuesday, April 27 Ch. 15.3 Atomic spectra Ch. 15.4 Lasers We are here Atomic spectra Bohr s atom Ground state, excited state Emission, absorption Spectral lines, astronomy Lasers 3 and 4 level lasers Population inversions Ruby laser, helium neon laser Solid state laser (laser pointer) Tunable dye laser 3 3 Photons and quantum theory Is light waves or particles? Light has wave properties of frequency and wavelength. Light travels in bundles called photons that carry a particular quantum of energy. Photons are countable, like particles. (Think Geiger counter). Photon cartoon 4

Photoelectric effect proves photons exist (Einstein 1905) UV photons hitting a metal surface release electrons. IR photons hitting the surface do not release electrons. This proves that the energy in photons is greater with higher frequency, or shorter wavelength. Photons hit a metal surface. Electrons leave a metal surface. 5 Einstein s formula 1921 Nobel prize E = h ν [Energy is proportional to frequency] E is the photon energy h = Planck s constant ν = frequency in radians per second = 2πf 6

Bohr s atom 1913 1. Niels Bohr proposed that negative electrons orbit the positive nucleus of the atom in specific numbered orbits with differing energy. 2. Emission of light (photons) occurs when electrons change orbit. 7 Bohr s atom emits a photon E 3 E 2 = hν Energy in level 3 energy in level 2 = energy of photon emitted 8

Energy levels Bohr s model was based on the vibrations of strings of musical instruments. Standing waves can have 1, 2, 3, etc. half wave lengths between the ends. Higher frequency = higher energy. Standing waves Demo: standing waves on rope 9 Emission and absorption Absorption of photons is emission in reverse. Photon energy = orbit 3 energy orbit 2 energy 10

Emission and absorption Chemical elements that are gases absorb when they are cold the same photons that they emit when hot. 11 Spectroscope The spectroscope is designed to create a bright spectrum of emission lines with colors located according to wavelength. slit 12

Absorption spectrum Chemical elements that are gases absorb when they are cold the same photons that they emit when hot. slit 13 Spectral lines Incandescent lamps emit a continuous spectrum with all colors (wavelengths). Elements from the periodic table emit specific wavelengths. These elements absorb at the same wavelengths they emit. Spectral line Hydrogen emission Hydrogen absorption 14

Chemical elements with more electrons have more complex spectra Hydrogen, 1 electron 6 2 5 2 4 2 3 2 Iron, 26 electrons Length of arrows is energy difference. 15 Role of spectroscopy in astronomy Distant stars have the same spectral lines as the sun, hence the whole universe has the same periodic table of elements! Some elements (helium, for example) were discovered first by spectroscopy. Bohr s atom explained why there were spectral lines in the spectrum of the sun and the chemical elements. 16

Spectrum of the sun The sun s cooler atmosphere absorbs light from below. 17 We are here Lec. 29, Tuesday, April 27 Ch. 15.3 Atomic spectra Ch. 15.4 Lasers Atomic spectra Bohr s atom Ground state, excited state Emission, absorption Spectral lines, astronomy Lasers Induced emission Population inversions Ruby laser, helium neon laser Solid state laser (laser pointer) Tunable dye laser 18 18

Induced emission A photon passing by an atom in an excited state will cause (induce) the excited electron to emit a photon and move to the lower state. The passing photon must have the same energy as the difference in the energy levels. start finish In phase! Same direction! 19 Population inversion A population inversion is a group of atoms with more electrons in an excited state than in the ground (lowest) state. An inversion is created by pumping some energy into the group of atoms, for example, an electrical discharge. 20

Light amplification by stimulated emission of radiation The population inversion results in the amplification of light passing through. Xe flashlamp can pump atoms to higher state. = 21 Laser cavity A cavity formed by two mirrors is used to make the light go in a single direction. The mirrors may be flat or curved. 22

Ruby laser (solid material) Ruby rod Ruby rod end view The rod ends are given a mirror-like coating. A flashlamp pumps the ruby rod with light. The beam comes out in a flash. 23 Helium neon laser (gas material) The pump is an electrical current passing through the gas. 24

Tunable dye laser (liquid material) The dye is pumped by a flash of light or another laser. The output can be tuned using a grating. Hence any wavelength can be obtained (within limits). A laser was made from dyed jello. 25 Laser pointer (laser diode) The lasing material is a semiconductor like silicon. The silicon has additives (dopants) that give it extra electrons (n type) or a deficit of electrons (p type). This creates energy levels with a band gap. Electrons crossing the band gap emit photons. The chemical composition can be varied to change the wavelength. Blu-ray compact disks use 405 nm blue laser light. The spots written on the disk are 580 nm across. Regular CDs use 780 nm red laser light. Track spacing is 1.6 μm. DVDs use 650 nm laser light. 26

World s most powerful laser http://www.youtube.com/watch?v=jy3jbc51wsk&nr=1 Fusion energy Hydrogen is turned into helium in the sun with a release of energy. The reaction requires that the fuel be heated to 100 million degrees. Scientists at Lawrence Livermore Laboratory attempt to do that with 192 focused laser beams. At the focus of the lasers is a thimble with a capsule of hydrogen. The heavy isotopes of hydrogen, deuterium and tritium, are used. The project is supposed to work some time in 2010. 27 Recent optical illusions at YouTube Illusion of a cube: http://www.break.com/index/optical illusion with candles.html Motion illusions: http://www.youtube.com/watch?v= KP KJdMp3U&NR=1 http://www.youtube.com/watch?v=t5xtrdloopu

Ch. 12 Wave optics 1. In phase, out of phase 2. Antireflection coatings, oil slicks, soap bubbles 3. Hard and soft reflections 4. Multilayer thin films 5. Newton's rings 6. Coherent source 7. Interference 8. Huygens principle 9. Iridescent colors 10. Slits, diffraction gratings 11. Two sources: what arrives depends upon source phase and the difference in the spacing 12. Finding the wavelength of light using diffraction: λ = S X / D. 31 Ch. 13 Scattering and polarization 1. Scattering: dependence on size of particles and on wavelength 2. Polarization. By reflection and scattering. 3. Polarizing filters. 4. Polarization in photography. What can it do? 5. Liquid crystal displays, how do they work? 6. Birefringence 32

Ch. 14. Holography 1. Holography apparatus 2. Diffraction patterns 3. Transmission holograms 4. Multichannel holograms 5. Cylindrical (integral) holograms 33 Ch. 8 Binocular vision 1. Why two eyes 2. Depth in art 3. Perspective 4. Making and viewing 3d images 5. Red/blue and polarizing glasses 34

Ch. 15 Spectroscopy and lasers Spectroscopy Photoelectric effect Atomic energy levels Photons Emission and absorption Spectral lines Lasers Induced emission Population inversion Laser cavity Types of lasers 35