Core Unit III: Light
B. Reflection

1). Laws of Reflection

Key Concepts

An interface is boundary region between two media.

A light ray is a stream of light with the smallest possible cross-sectional area. (Rays are theoretical constructs.)

The incident ray is defined as a ray approaching a surface.

The point of incidence is where the incident ray strikes a surface.

The normal is a construction line drawn perpendicular to the surface at the point of incidence.

The reflected ray is the portion of the incident ray that leaves the surface at the point of incidence.

The angle of incidence is the angle between the incident ray and the normal. The angle of reflection is the angle between the normal and the reflected ray.

Laws of reflection

• The angle of incidence is equal to the angle of reflection.
• The incident ray, the normal, and the reflected ray are coplanar.

Specular reflection (regular reflection) occurs when incident parallel rays are also reflected parallel from a smooth surface.

If the surface is rough (on a microscopic level), parallel incident rays are no longer parallel when reflected. This results in diffuse reflection (irregular reflection).

The laws of reflection apply to diffuse reflection. The irregular surface can be considered to be made up of a large number of small planar reflecting surfaces positioned at slightly different angles.

Indirect (or diffuse) lighting produces soft shadows. It produces less eye strain than harsher, direct lighting.

A sharp contrast between highlights and shadows is produced by direct lighting.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: interface, ray, incident ray, point of incidence, normal, reflected ray, angle of incidence, angle of reflection, specular reflection, diffuse reflection, indirect lighting, direct lighting.

2. State the laws of reflection.

3. Compare and contrast specular and diffuse reflection.

4. Explain why the laws of reflection still apply for diffuse (irregular) reflection.

5. List some different kinds of surfaces which produce either specular or diffuse reflection.

6. Compare the effects produced by direct and indirect lighting.

7. Apply the laws of reflection in problem solving.

Teaching Suggestions, Activities and Demonstrations

1. Illustrate the different kinds of effects produced by direct and indirect lighting. Compare fluorescent lights (perhaps like the ones in your classroom) with incandescent bulbs. Also, show the different effects produced by a clear incandescent bulb and a "soft" coated incandescent bulb.

2. Give some examples which illustrate the appropriate use of direct and indirect lighting.

3. Determine some of the ways that diffuse or direct lighting are produced from artificial light sources.

4. Draw or label a diagram illustrating specular reflection of a single light ray.