Ray Optics: Tracing the Path of Light Using a Laser

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Author: Jerry Silver

The Idea

This is a perfect way to see for yourself how light moves when it encounters mirrors and lenses.

What You Need

  • laser pointer
  • set of lenses, including convex, concave, rectangular, and semicircular lens; rectangular prism and 60 and right-angle prisms (90°, 45°, 45°) and (90°, 30°, and 60°)
  • 2 small flat mirrors
  • sheet of paper (plain or gridded)
  • ruler
  • protractor
  • dark room


  1. Caution—You should use a low-power laser pointer and be careful not to shine the laser where it could hit anyone's eyes. Remember, you are working with optical devices that change the path of the light, so be careful to avoid stray light rays that could affect anyone's eyes.
  2. Place your object lens (or mirror) on a flat table.
  3. Place a sheet of paper underneath the lens.
  4. Trace the outline of the lens on your paper. Leave enough room to draw incoming and outgoing lines.
  5. Darken the room.
  6. Shine the laser at a slight angle, so its straight line path can be seen on the paper.
  7. For each of the lenses, put three or more dots along the path to define the incident path.
  8. Observe its path through the lens (or reflected from the mirrors). You may need to slightly adjust the angle (to the plane of the table) to make the transmitted ray visible. Depending on your lenses, you may not be able to see the laser light going through the lens. Also, be careful not to mistake light that may sneak underneath the lens as a ray that follows the intended optical path through the lens. Also (again depending on your lenses), you may need a slightly different angle to make the incident laser line visible as you would need for the refracted line. If this is the case, make sure you come into the lens along the same incident line that you drew.
  9. Make three or more dots to define the refracted (or reflected) paths.
  10. Make a dot where the light enters the lens and where it leaves the lens.
  11. Connect the dots with straight lines showing the incident (incoming) line, the straight line through the lens (which is the refracted line), and the transmitted or reflected lines.
  12. Explore as many of the following optical objects as you have available. The following lists several specific things to focus on.

(Note: All this can be done, if you prefer, on a magnetic chalkboard using lenses with magnetic backs. You can either glue strong magnets to your lens or simply hold the lens to the chalkboard. Make sure that the magnetic chalkboard is strong enough to hold the lens securely and that the magnet does not block the path of the light. Laser levels may be useful because they have built-in angles to make the line visible along a surface. However, they may be a little trickier to focus all the way through the lens.)

Single mirror

Draw a perpendicular line to the surface of the mirror. Shine the laser at the point where the perpendicular line meets the mirror. Place dots along the incident line and the reflected line, and then connect the dots. Compare the incident angle with the reflected angle. Try this for several sets of angles (Figure 78-1).

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