Introduction
Lasers have become ubiquitous in recent years, from medical facilities and classrooms to check-out counters. What causes a laser to produce laser light? Model the stimulated emission of a helium–neon (He–Ne) laser, and show why lasers are very inefficient.
Concepts
- Stimulated emission of light
- Monochromatic light
- Lasers
- Collimated light
Background
LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. The expression LASER is so commonplace now that the acronym is also considered to be a word, so “laser” is typically written in lower-case. In this demonstration, the BBs represent the monochromatic (one color—one wavelength) photons bouncing back and forth in a laser tube. (The photons are generated by stimulated emission.) One end of the tube is 100% reflective, while the other end has a small opening and is considered to be “99%” reflective. Shaking the model “laser” simulates what happens to the reflecting light—only light reflected parallel to the tube length (and perpendicular to the mirror ends) will continue to reflect, while any stray light will be absorbed. Since only one wavelength of light is stimulated in a He–Ne laser, only one photon wavelength (one color BB) is present. The opening at the end will let out one BB at a time and only if the BB enters the tube with the proper energy (speed) and orientation (parallel to the tube). Shaking the tube releases only a few BBs at a time, showing that lasers are very inefficient. When the BBs are released, they travel in the same direction initially, simulating collimated light.
Materials
BBs, 150*
Demonstration tray, large
Plugs, foam, 2*
Ruler
Scissors
Straw, drinking
Tube, clear, plastic*
*Materials included in kit.
Safety Precautions
The materials in this demonstration are considered non-hazardous. Collect any BBs that may have rolled onto the floor to prevent a person from slipping on them.
Disposal
The materials should be stored and saved for future use.
Prelab Preparation
- With scissors, cut off a 2" piece from a drinking straw.
- Insert a flat end of one foam plug into one end of the plastic tube. The inserted end should be parallel to the opening of the tube (see Figure 1).
{12619_Preparation_Figure_1}
- Carefully fill the plastic tube with approximately 50 BBs.
- Insert the 2" straw piece at the open end of the tube, and then insert the flat end of the second foam plug into the tube to secure the straw in the tube. The inserted end of the foam plug should be parallel to the opening of the tube. The ends of the straw should be free from obstructions. See Figure 2 for the completed “laser model.”
{12619_Preparation_Figure_2}
Procedure
- While explaining the theory behind an operating laser, show students the model laser.
- Place the model laser in a large demonstration tray or other shallow container. If a clear tray is available, the model laser may also be used on an overhead projector. The shallow tray is optional, but it will help reduce the loss of the BBs exiting the “laser” during the demonstration—they run all over.
- Rotate the tube so the straw is on the bottom (at the same level as the BBs in the tube).
- Gently shake the tube back and forth in a direction parallel to the length of the tube. Watch as the “photons” are emitted at the straw end of the model laser.
- Continue to shake the model laser as necessary during the laser theory explanation.
Teacher Tips
- Enough materials are supplied to perform this demonstration indefinitely. Collect and store the BBs in the zipper-lock plastic bag when finished with the demonstration.
- Place the simulated laser in a large demonstration tray to prevent the BBs from rolling all over the classroom.
- For further technical information on lasers, see the “Laser Theory,” Flinn Publication No. 10426, included with this kit. Copy this publication for student use, if desired.
- The Laser Model Worksheet may also be copied and used during the discussion or as a post-demonstration assignment, if desired.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Developing and using models
Disciplinary Core Ideas
HS-PS1.A: Structure and Properties of Matter
HS-PS3.A: Definitions of Energy
Crosscutting Concepts
Systems and system models
Performance Expectations
HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Answers to Questions
- Define the acronym LASER.
LASER is the acronym for Light Amplification by Stimulated Emission of Radiation.
- What do the BBs represent in this demonstration?
The BBs represent the monochromatic stimulated light produced in the laser.
- Why does a He–Ne laser produce only one wavelength of light (monochromatic)?
Only one energy transition exists between the second exited state of neon and the first excited state. When the excited electrons “fall” from the second to first excited energy levels, each photon of light that is emitted will have the same energy and wavelength. The helium atom provides the energy to “pump up” the neon atom to its second (meta) excited state.
- Explain the term collimated as it refers to laser light.
Laser light is collimated, meaning the beam of light is parallel to the laser tube and remains parallel for a very long distance.
- Why is a He–Ne laser inefficient?
The laser light reflects back and forth many times within the tube, which helps amplify the number of photons emitted through stimulated emission. However, only about 1% of the light exits the “open” end of the laser.
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