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Optical Alignment: Exploring Law of Reflection

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Author: Jerry Silver

The Idea

This is a simple and fun way of exploring the law of reflection that provides an initial insight into what is needed to achieve optical alignment. This project (which I first heard from Tom Misniak) makes a good team-building activity and can be used as the basis for a friendly competition.

What You Need

  • low-power laser (a laser pointer is fine)
  • apparatus (such as a tripod) to mount the laser point and hold it illuminated and stationary for a sustained period (you may need to tape the laser point to keep it on without holding it)
  • several plane mirrors
  • small white boards
  • way to mount the mirrors, such as ring stands and clamps or modeling clay
  • dark room
  • timer

Method

Competition 1 (Round robin)

  1. Caution: Be careful not to shine the laser beam at anyone's eyes. Although the power of the laser should be low, it is a good idea to take care not to expose anyone's eyes.
  2. Distribute a mirror and associated mounting hardware to each participant.
  3. Draw a target and mount it in a location where everyone has a clear line-of-sight to it.
  4. Set up the laser pointer in a central location.
  5. Place the participants at different locations around the room.
  6. Define a sequence for the beam to reflect from one person to the next and, finally, to the target.
  7. Optional: darken the room.
  8. Direct the beam to the first mirror. Use the white board, if necessary, to "capture" the beam.
  9. Optional: start the timer.
  10. Next, align the pointer and the first mirror to make the beam reflect to the second mirror.
  11. Continue from one mirror to the next until the last mirror directs the beam to the target, as shown in Figure 80-1.
  12. Optional: in case this gets too easy, require the final beam to pass through a cardboard tube mounted in front of the target.
  13. Compasses (for Competition 2).

Laser Obstacle Course

Competition 2: Back to the target

  1. Take similar precautions, distribute mirrors, and establish a target, as in Competition 1.
  2. Place the participants at different locations around the room.
  3. This time, however, have each participant work out their angles and alignment based on applying the law of reflection and measuring angles.
  4. Give each participant a set time.
  5. When the time is up, all participants must no longer touch the mirrors. In fact, you can have them leave the area altogether.
  6. Then comes the moment of truth, where you see how close each of the participants is able to direct the laser to the target.

Expected Results

It is not unreasonable to have six reflections in about ten minutes. This requires interaction and coordination between groups. One lesson people doing this learn is that small changes at the beginning of the course result in large errors at the end. Even vibrations in the first mirror in the sequence can throw off the alignment downstream. Adjustments may need to be made at each step. Another valuable lesson is there comes a time when it is best to leave the mirror alone and stop making changes.

One other thing that may not immediately be obvious is this is a three-dimensional alignment problem. You not only need to adjust left and right, but also up and down.

Why It Works

This is an application of the law of reflection. Because the reflection angle from one mirror becomes the incident angle of the next mirror, errors in incidence double with each reflection.

Other Things to Try

For the truly dedicated: work out the angles for multiple reflections and back to the target on paper first.

The Point

The angle of incidence equals the angle of reflection. Small alignment errors can be quickly compounded when multiple reflections occur.

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