The Physics of LASERs - American Physical Society

Teacher Edition Lesson 2: How Does a LASER Work?
Guidelines for variable settings:
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• Use the Multiple Atoms (Lasing) tab
• Select three energy levels
• Maximize level two’s lifetime
• Match lamp output energy to energy between
levels three and one
• Enable mirrors, and set mirror reflectivity to
96%
• Adjust slider for lamp intensity so that level
two’s population is maximized
Image credit: PhET Interactive Simulations, University of
Colorado, http://phet.colorado.edu
Image credit: PhET Interactive Simulations, University of
Colorado, http://phet.colorado.edu
Part B, Step 3
An easy way to achieve this is to have the same laser
settings as those that worked for Step 2, but now
make both mirrors 100% reflective.
Part B, Step 4
Students should indicate that an energy source is
needed to initially excite atoms that make up the active
laser medium. The active medium transforms
the incident input energy into the laser output light
energy. Mirrors are needed to collimate this light
energy and allow it to build up. From the PhET
simulator students should recognize that the active
medium is composed of atoms. The atom’s energy
levels used in the lasing process must have a long
enough lifetime to allow for stimulated emission to
The Physics of LASERs, Teacher Edition
Student Edition Lesson 2: How Does a LASER Work?
B. What conditions do you need to get the Phet simulator laser to work?
You are going to try to get the PhET simulator to lase (produce laser light). You will
need more than one atom, so use the PhET simulator with the multi-atom tab selected.
1. With your group decide on your procedure to make this computer model of a laser
work (lase). List all of the variables. For each one determine an initial setting, and
why you chose it.
2. As you go through your procedure, record your observations. How do the variables
you change affect the lasing process? Record any changes you make and why you
made them.
3. What could you do to get the PhET simulator into the danger zone? (See the laser
power indicator). Watch out, the laser could blow up!
4. What conditions do you need to get a laser to work? Using what you have learned
from the PhET simulator, describe how you could make a working laser.
Student Version Page 10
occur. Students might have a sense that it is easier for this process to
occur with multi-level atoms making up the active medium.
Did students reach the goal of Part B? Students should have adjusted
the variables in the PhET simulator using the multi-atom, threelevel
settings, to obtain the lasing process. Their answer in Part B,
Step 4 should describe the conditions they needed to get the PhET
simulator to lase.
Image credit: PhET Interactive Simulations, University of
Colorado, http://phet.colorado.edu
The Physics of LASERs 11