In questions 18-31, you wil work through the requirements needed to make a laser
ID: 2269119 • Letter: I
Question
In questions 18-31, you wil work through the requirements needed to make a laser. You will be working with the Laser Simulation to really discover and understand the requirements and operational considerations yourself: http://phet.colorado.edu/en/simulation/lasers (or go to course website) (18-20) A laser involves three processes by which light interacts with atoms: absorption, spontaneous emission, and stimulated emission. Which of the following descriptions accurately describes each process (absorption, spontaneous emission, and stimulated emission): 18. (0.5 Points) The process in which the electron naturally jumps down from a higher energy state to a lower state and spits out a photon corresponding to the energy difference as it does so: 19. (0.5 Points) The process by which the light is absorbed and the energy causes the atomic electron to go to a higher energy level: 20. (0.5 Points) The process where a photon hits an atom that is in a higher energy level already and this causes the atom to spit out a photon that is identical to the one that hit the atom so there are two identical photons: 21. (0.5 Points) A laser is created by producing a lot of photons created by stimulated emission. What characteristics make the light produced by stimulated emission so special compared to light produced through spontaneous emission? (Choose all that apply) A. Photons are traveling exactly in the same direction. B. Photons' electromagnetic waves are oscillating exactly in phase C. Photons are all exactly the same color. D. Photons have more energy
Explanation / Answer
18) Spontaneous emission
19) Absorption
20) Stimulated emission
21) A, B and C. Stimulated emission gives photons that are in the same direction, coherent (same phase) and the same energy.
22) A. For the absorption to occur in a system of two levels, the incident photon must have energy equal to or greater than the energy difference between the two levels.
23) True. In stimulated emission, one photon entering the system will stimulate one more photon to be emitted hence giving out two photos. This is a net gain in the number of photons.
24) False. In stimulated emission, the direction of the emitted photon is in the same direction as the stimulating photon.
25) True. In spontaneous emission, the direction of the emitted photon is random.
26) Decreasing the lifetime of upper energy level will decrease the average amount of time before an excited atom will undergo spontaneous emission, increase the likelihood of spontaneous emission and decrease the likelihood of stimulated emission.
27) For higher stimulated emission, increase the lifetime of the excited state and increasing the lamp intensity.
28) More than half the population of atoms in the cell need to be in the excited state for a net increase in the number of photons. A net increase in the number of photons is possible only through stimulated emission which requires atoms to be in its excited state. So we need more number of atoms in the excited state for a gain in the number of photons.
29) If we shine a sustained high-intensity beam, half the atoms will be in the lower state and the other half in the excited state. This is the condition in thermal equilibrium.
30) We need a minimum 3 atomic levels to make a laser. This is because we need a state of population inversion, i.e., more than half the atoms to be in the excited state at all times for sustained lasing activity. A 2 atomic level system will at max have an equal number of atoms in lower and excited state at equilibrium, which will not give gain. The process of exciting the atom by absorption will eventually be in equilibrium with spontaneous and stimulated emission and lasing will die off.
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