Introduction
Lasers produce a very narrow beam of almost monochromatic light. This beam of light remains collimated over long distances. Reflecting this beam off of two spinning mirrors creates beautiful patterns commonly called Lissajous figures. It’s hard to imagine something so beautiful could be created from such a simple demonstration apparatus!
Concepts
- Reflection
- Lissajous figures
Materials
Bracken demonstration spinners, 2
Laser light show board*
Laser pointer
Round mirror and attachment assembly, 2*
Velcro® strips, hook, 2*
Velcro strips, loop, 2*
*Materials included in kit.
Safety Precautions
Please follow the safety guidelines described by the laser’s manufacturer. Never look directly at the laser light. For further safety information, please see The Safe Use of Lasers write-up included in this kit.
Disposal
Materials may be saved and used for future demonstrations.
Procedure
- Obtain two Bracken demonstration spinners (Flinn Catalog No. AP6202) and the two round mirror and attachment assemblies. Place one attachment assembly onto each of the demonstration spinner axles (see Figure 1).
{13915_Procedure_Figure_1}
- Remove the backing and place a strip of “hook” Velcro on the bottom of each demonstration spinner as shown in Figure 2.
{13915_Procedure_Figure_2}
- Remove the backing and place strips of “loop” Velcro on the laser light show board as shown in Figure 3.
{13915_Procedure_Figure_3}
- Place the two Bracken demonstration spinners on the laser light show board as shown in Figure 4.
{13915_Procedure_Figure_4}
- Darken the room and obtain a laser pointer. Adjust the placement of the Bracken demonstration spinners so that the laser beam will be reflected off the first mirror and onto the second mirror as shown in Figure 4.
- Hold the laser light show board in hand and reflect the laser image onto a wall or screen.
- Start one of the demo spinners by plugging in the two wires. Reflect the laser beam off of the first spinning mirror. The appearance of an elliptical shape will be reflected onto the wall.
- When the second motor is started, and the laser beam is reflected off of both the rotating mirrors, beautiful Lissajous patterns will result.
- The pattern on the wall or screen can be changed by simply varying the speed of one of the motors. Do so by tapping or placing your finger on one of the rotating mirrors to slow them down slightly.
Teacher Tips
- This kit contains enough materials to perform the demonstration as many times as needed.
- Red lasers such as Flinn Scientific’s Pocket Laser Pointer (Catalog No. AP4667) or Laser Pointer (Catalog No. AP8934) will work well with this activity, however, green laser pointers (Catalog No. AP6492) produce a much stronger beam and clearer Lissajous images.
- It is best to have a completely dark room for this demonstration. The resulting laser image is best seen if it is projected onto a screen or blank wall.
- The further away the laser light show board is from the screen, the larger the image will be.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Developing and using models
Disciplinary Core Ideas
MS-PS4.B: Electromagnetic Radiation
HS-PS4.B: Electromagnetic Radiation
Crosscutting Concepts
Patterns
Stability and change
Performance Expectations
MS-LS2-4: Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.
MS-ESS3-3: Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment.
HS-ESS3-6: Use a computational representation to illustrate the relationships among Earth systems and how those relationships are being modified due to human activity.
Discussion
In this demonstration, a laser beam is reflected off two rotating mirrors. As the laser beam is reflected off the first single rotating mirror, an oval pattern is projected. When the laser beam is reflected off both of the spinning mirrors, beautiful patterns commonly called Lissajous figures are seen.
Lissajous figures (also known as Bowditch curves) were discovered by the French physicist Jules Antoine Lissajous in 1857. Lissajous would use tuning forks to produce sounds of different frequencies to vibrate mirrors. A beam of light was then reflected from the mirrors and different projected patterns could be seen, depending on the frequencies of the sounds.
Lissajous’ original setup was very similar to the ones used today in professional laser light shows. Professional laser light shows often use computerized systems to control the shapes and sizes of Lissajous figures. These figures can then be used to show animated sequences of shapes that zoom, rotate, spin and can even be synchronized with music. Lissajous figures are also used to determine the frequencies of sounds and/or radio signals. A signal of known frequency is applied to the horizontal axis of an oscilloscope. The signal to be measured is then applied to the vertical axis. The resulting pattern is a result of the function of the ratio of the two frequencies.
Lissajous figures are produced by taking two sine waves and displaying them at right angles to each other. The actual shape projected from the two spinning mirrors depends on the frequencies, or oscillations per second, of both curves. The frequencies of the curves are in whole number ratios to one another such as 1:1, 1:2, 1:3 and so on. If the difference in the whole-number ratio is significant, the phenomenon of shape shifting occurs. See the Lissajous figure table (see Figure 5).
{13915_Discussion_Figure_5_Lissajous figure table}
For further information on the theory and applications of lasers please see the Laser Theory Physical Science Fax included in this kit.
References
Ballegeer, D. J., Drumheller, J. E., Kirkpatrick, L. D., Rugheimer, M. Phys. Teach., 1982, 20, 415.
Chiaverina, C. Phys. Teach., 1990, 28, 606.
Pinkerton, K. D. Phys. Teach., 1991, 29, 168–169.
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