Reflection and Reflecting: The Law

Law of Reflection

Light behaves in a very predictable way. If a ray of light is observed approaching and reflecting off of a flat mirror, then the behavior of the light acts in a predictable manner. The diagram below illustrates the Law of Reflection.

Law of Reflection

In the diagram, the ray of light approaching the mirror is known as the incident ray. The ray of light which leaves the mirror is known as the reflected ray .

At the point of incidence where the ray strikes the mirror, a line can be drawn perpendicular to the surface of the mirror. This line is known as a normal line (labeled N in the diagram).

The normal line divides the angle between the incident ray and the reflected ray into two equal angles. The angle between the incident ray and the normal is known as the angle of incidence.

The angle between the reflected ray and the normal is known as the angle of reflection.

The law of reflection states that when a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection.

Inquiry-Based Investigation

Materials: 30 cm of aluminum foil per student

Procedures:

1. Students lay the aluminum flat on a table, shiny side up.
2. Students lean over the aluminum foil to a point where they see their reflection and record their observations.
3. Students wrinkle the aluminum foil (however not too much).
4. Students now repeat step 2 and record their observations.

Student Questions:

• What did you see in the aluminum foil in step 2?

• What did you see in the aluminum foil in step 4?

• Why do you think you no longer see your reflection in step 4?

• What would you have to do so that you can see your reflection again in step 4?

Other questions based on student responses to these questions. Twenty questions to ask students provides additional examples of types of questions to ask students.

Types of Reflection

Reflection off of smooth surfaces such as mirrors or a calm body of water leads to a type of reflection known as specular reflection.

Reflection off of rough surfaces such as wrinkled aluminum foil, clothing, and paper leads to a type of reflection known as diffuse reflection.