This week’s lab is using an older simulationLinks to an external site. from PhET
This week’s lab is using an older simulationLinks to an external site. from PhET. Yes it takes a while to load, yes it is also a bit buggy. If it gets stuck, reload the page. If you are having any trouble getting it to run, please send me a message via Canvas.
This lab is really cool though, because LASERs.
OpenStax has some help information in chapter 17.2: Wave Based Applications of Light.Links to an external site.
Get the simulation running.Seriously, if you have any issues, send me a message asap.
Navigate to the “One Atom (Absorption and Emission)” tab.
Without changing any other settings, slightly turn on the top slider from “Lamp Control.”You want about 1 photon emitted per second.
Observe what happens for a bit.Question 1: When the photon hits the unexcited (gray) atom, what happens to the photon? Where does its energy go?Hint for where the energy goes: look at the chart in the upper right corner.
Question 2: What happens when the photon hits the atom while the atom is excited (red)? Is there any special type of photon emission that you can observe in this simulation?If the excited atom is decaying and emitting its photon randomly before the photon from the lamp can get to it, then just increase the intensity of the lamp (slide the top slider under “Lamp Control” slightly to the right) until photons are hitting the already excited atom.
In the upper right corner, slide “lifetime” all the way to the left. Question 3: How did this effect the simulation? Is there anything new regarding the direction of the photons emitted when the atom de-excites?
Slide the energy level up or down a bit, such that it does not line up with the squiggly red line in the upper right corner.Question 4: What behavior of the simulation changed? Why?
Slide the color slider (second slider in the Lamp Control) to change the color of the light until the atom begins to absorb/emit photons again.Question 5: What requirement is there (in terms of the energy levels in the upper right box) such that the atom will absorb and emit the photons?
Click “Reset All” and “Yes.”
Set Energy Levels to “Three” and turn on “Display photons from upper energy state.”
Turn on the blue lamp.Question 6: Describe in detail what is happening.
Slide the blue life all the way down (left) such that when blue photons hit the atom they are immediately de-excited to the red level.
Turn on the red lamp so that it is putting out a photon about once per second.Question 7: Again describe what is happening in the simulation. Do the emitted red photons behave similar to question 2?
This setup approximately describes how lasers work, except that lasers use the emitted red photons and mirrors to cause the stimulated emission observed above instead of two lamps. To set up a “laser” with one atom is tough thought, lets give it a try.
Turn the blue lamp intensity all the way up (slide to right, such that photons are coming out quickly).
Wait until a photon from the red light stimulates the emission of the same type of photon from the atom.
As that pair of atoms are moving to the right, turn on the mirrors by checking the “Enable Mirrors” box.
Question 8: Let it run for a while. What do you observe? You can submit a screenshot of your “lasing” set up as an answer for this question if desired.
Slide mirror reflectivity down to 90-95% and observe for a while.Question 9: What do you observe?
Extra Credit [1 point]
Switch to the “Multiple Atoms (Lasing)” tab of the simulation.
Use what we observed above to create a laser.
Submit a screenshot of your laser running for a bonus point.