The Physics of LASERs - American Physical Society

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Let students know that when an electron moves from one allowed energy level to another it is called a transition. The pattern of an electron’s allowed energy transitions are like fingerprints. Each element has its own distinct pattern of transitions. This pattern is used to determine what elements are present in stars, gases, and other substances. Draw or project the first seven atomic energy levels of hydrogen on the board (see drawing on previous page) and show the five transitions from state “2” in the drawing (this is also called the first excited state). Show the students a color version of the spectrum (“fingerprints”) of hydrogen, or even better, have students observe excited hydrogen gas through diffraction gratings. What are the colors of visible light in hydrogen’s “fingerprint?” Point out the colors of the absorption and emission spectra and the transitions that they correspond to on the energy level diagram (Balmer Series: red, cyan, blue, violet) Consider letting students know that an atom’s electrons are only allowed in certain energy levels, and these energy levels are discrete or quantized. Scientists usually express the energy levels of an atom in units of electron-volts (eV). 1eV = 1.602x10 -19 Joules. Students don’t need to know the details of electron volts for this lesson, just that it is a unit of energy, Teacher Edition Lesson 2: How Does a LASER Work? which is also used in the PhET simulator. Discuss absorption: when an atom’s electron moves to a higher energy level, the atom absorbs energy, and increases its internal energy. This energy could be transferred to it by the absorption of a photon (light energy) of just the right wavelength (or frequency). Energy is conserved–the light energy was transferred to the atom, absorbed and transformed, increasing the atom’s internal energy, as shown in the first two columns of the diagrams below. Display the first two columns below for the class Discuss spontaneous emission: When an atom naturally decays to a lower energy state, it is called “spontaneous decay” or spontaneous emission. In this process some of the atom’s internal energy is transformed into light energy. The light energy is emitted as the atom’s electron moves from a higher energy level to a lower one. The emitted photon has a frequency proportional to the energy difference between the atom’s two energy levels involved in the transition. Emphasize that the emitted photon may travel in any direction. The amount of time it takes for the atom to decay depends on the difference between the energy levels involved in the transition. The greater the energy difference, the faster the atom transitions to a lower energy state. Absorption and Spontaneous Emission of a Photon by an Atom photon approaches an atom atom has absorbed photon atom has emitted a photon photon energy = E 2 - E 1 photon Energy of atom A photon with energy approaches an atom. The atom is in its lowest energy level. photon energy = E 3 - E 1 photon Energy of atom E3 E2 E 1 E 3 E 2 E 1 atom _____3 _____2 _____1 Energy of atom The atom has absorbed the photon. The photon energy has been transformed into internal energy of the atom. Note that the photon is absorbed only if its energy is equal to the difference in energy of the atomic levels involved in the transition. E 3 E 2 E 1 photon energy = E 2 - E 1 atom atom _____3 _____2 _____1 The atom has returned to the lowest energy level and has emitted a photon. The photon may be emitted in any direction. The photon’s energy is equal to the difference in energy of the atomic levels involved in the transition. This process is called spontaneous emission. photon energy = E 3 - E 1 atom atom atom _____3 _____2 _____1 Energy of atom E 3 E 2 E 1 _____3 _____2 _____1 Energy of atom Energy of atom E 3 E 2 E 1 E 3 E 2 E 1 _____3 _____2 _____1 _____3 _____2 _____1 The Physics of LASERs, Teacher Edition 21
Teacher Edition Lesson 2: How Does a LASER Work? Kinesthetic model for absorption and spontaneous emission 22 • explain model • run model for absorption only • run model for absorption and spontaneous emission • discuss stimulated emission • run model again with stimulated emission. Let students know that in this model • students participating in the model will represent energy, either as photons (packets of light energy) or as internal energy of the atom. • a piece of paper will represent the atom. When the paper is on the ground the atom is in its lowest energy or ground state. • the photons all have just the right amount of energy to be absorbed by an atom, causing it to move to a higher energy state. • to express the absorption of the photon, and the increase in the atom’s internal energy, the student stops moving and lifts up the paper (atom in an excited state). • two walls (or desks) represent mirrors creating the laser cavity. (Atomic “states” correspond to atomic energy levels.) photon energy = E 2 - E 1 photon energy = E 3 - E 1 The Physics of LASERs, Teacher Edition Stimulated Emission Run the kinesthetic model to simulate absorption of photons • Have a cleared area on the floor, preferably where students had performed the kinesthetic models during Lesson 1. • Place some sheets of construction paper on the floor. • Let the students know that the construction paper on the floor represents atoms in their ground state. • Then have a few students be photons. Label the students with different colors of light. • Let the class know that only photons that are red are just the right frequency (meaning they carry just the right energy) to be absorbed by the atoms in this model. Then have photons travel to the atoms. All the photons but the red ones should walk straight to the other side of the room. The red photons should stop at the atom and lift up the construction paper to simulate an atom absorbing a red photon and increasing in internal energy. If some red photons forget and walk straight through that is okay. Let the class know that realistically, not all the red photons would be absorbed by the atoms. Remind students that the students represent energy in the model. Initially all students are photons (light energy). photon approaches an excited atom atom emits a photon photon atom A photon approaches an atom that is already in an “excited state”–in this case, in energy level 2 (above) or energy level 3 (below). Note that the energy of the emitted photon is equal to the difference between the initial energy of the atom and its final energy photon Energy of atom atom Energy of atom E3 E2 E 1 E 3 E 2 E 1 _____3 _____2 _____1 _____3 _____2 _____1 photon energy = E 2 - E 1 atom The atom emits a photon by “stimulated emission.” This photon has the same energy as the photon that approached the atom, and also the same phase (the photons are in step). The internal energy of the atom changes into the energy of the emitted photon. photon energy= E 3 - E 1 atom 2 photons 2 photons Energy of atom Energy of atom E 3 E 2 E 1 E 3 E 2 E 1 _____3 _____2 _____1 _____3 _____2 _____1
Page 1 and 2: Teacher Edition The Physics of LASE
Page 3 and 4: 2 Table of Contents Teacher Introdu
Page 5 and 6: Materials Complete Kit List 4 Teach
Page 7 and 8: 6 Teacher Introduction Models are o
Page 9 and 10: 8 Lesson 1. What’s Special about
Page 11 and 12: Teacher Edition Lesson 1: Propertie
Page 19 and 20: 18 Lesson 2. How Does a LASER Work?
Page 21: Teacher Edition Lesson 2: How Does
Page 25 and 26: Teacher Edition Lesson 2: How Does
Page 31 and 32: 30 Lesson 3. What are Some Applicat
Page 33 and 34: Teacher Edition Lesson 3: What are
Page 41 and 42: Appendix 1 40 Sine Waves

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