Ray Optics: Tracing the Path of Light Using a Laser (page 3)

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Author: Jerry Silver

Expected Results

Single mirror

The measurements should validate the idea that the angle of incidence equals the angle of reflection.

Ray Optics Tracing the Path of Light Using A Laser

Mirrors at a right angle

Regardless of the angle at which the laser ray strikes the mirror, the ray reflected from the second mirror will be parallel to the incoming ray, but heading in the opposite direction.

Convex lens

Rays striking the lens traveling along the centerline will go straight through the lens without being diverted. Rays traveling parallel to the centerline bend toward the centerline and cross at a point (called the focal point) on the opposite side of the lens.

Ray Optics Tracing the Path of Light Using A Laser

Rays passing through a focal point on the same side of the lens as the laser (same distance as the focal point measured previously) emerge from the lens parallel to the centerline.

Concave lens

No matter where the rays hit the lens, the rays do not cross. Above the centerline, the rays bend up. Below the centerline, the rays bend down.

Semicircular lens

Rays hitting the semicircular side and directed toward the center of the circle bend (or refract) only when emerging from the glass to the air.

Rectangular prism

Rays emerging from the lens are parallel to the incident ray, but offset in the direction that rays move through the lens. As with all these observations, the path the ray takes in the prism is a straight line.

Right-angle prism

Rays striking one of the two shorter edges of the (90°, 45°, 45°) prism make a 90-degree turn, and then reflect back.

Rays striking the longer (hypotenuse) edge of the (90°, 30°, 60°) prism make a 180-degree turn, and then reflect back.

60-degree prism

These produce a range of transmitted angles and conditions for total internal reflection.

Why It Works

Lenses are optical devices that refract light passing from one medium (air) through another (glass or plastic), and then typically back to the air. At each interface, the light is bent according to Snell's law.

Mirrors, whether involving one or many surfaces, reflect light in such a way that the angle of incidence equals the angle of refraction.

Other Things to Try

The lenses and mirrors previously described can also be studied using the following additional methods:

  • Ray tracing with split beam. You can make or buy an apparatus that projects several parallel beams of light. Basically, the apparatus consists of a bulb covered by an enclosure that shines through parallel openings in the side of the enclosure. The beams of light, when directed at the various lenses and mirrors, show the properties of the devices in a graphic and intuitive way. This approach is better suited to smaller lenses and mirrors.
  • Ray tracing by locating images. A more traditional approach is to view a vertical object such as a pin or a nail through the lens. This is accomplished by: a) tracing the outline of the lens, b) locating the position of the image seen through the lens, and c) drawing a line to show the incident, refracted, and transmitted paths of light.

The Point

Lenses and mirrors divert light in predicable ways. Lenses are based on refraction, while mirrors employ reflection. Some lenses are converging and direct rays of light passing through them through a focal point. Other lenses are diverging and direct the rays of light in such a way that they never cross. The reflection at the surface of any plane mirror occurs in such a way that the angle of incidence equals the angle of reflection.

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