The Laws of Reflection

11.6 The Laws of Reflection

Overview of Reflection Laws

  • When an incident light ray strikes a plane mirror, it reflects to form a reflected ray.

  • These behavior patterns of light lead to the two fundamental laws of reflection:

    1. Angle of Incidence: The angle of incidence (B8i) equals the angle of reflection (B8r).

    2. Planarity: The incident ray, the reflected ray, and the normal to the mirror's surface all lie in the same plane.

Setup for Activities

Equipment and Materials

  • Flashlight (or an unshielded ray box)

  • Sheet of aluminum foil (30 cm x 30 cm)

Activity 1: Observing Reflection with Flat vs. Crumpled Foil

  1. Preparation: Place a smooth piece of aluminum foil flat on a table in a darkened room.

  2. Experiment: Shine a flashlight or ray box on the foil to reflect the beam onto the ceiling.

  3. Observation: Examine the shape of the reflected beam on the ceiling.

  4. Modification: Crumple the aluminum foil and open it up without smoothing it.

  5. Repeat: Reflect the light beam off the crumpled foil and examine the new shape of the reflected beam.

  6. Questions for Reflection:

    • A. Describe the shape of the reflected beam when the foil was flat versus crumpled.

    • B. Account for the differences in the reflected beam shapes between the two trials.

Reflecting Light Off Surfaces

  • Multiple incident light rays striking a flat reflective surface can lead to various outcomes based on the surface's quality.

  • Specular Reflection:

    • Occurs when parallel incident rays strike a smooth, shiny surface (e.g., plane mirror, calm water, flat aluminum foil).

    • All rays reflect at identical angles, resulting in parallel reflected rays.

Examples of Specular Reflection

  • Reflection off a plane mirror (Figure 2).

  • Reflection off calm water (Figure 3).

  • Disco mirror balls reflecting light in various directions (Figure 4).

Diffuse Reflection

  • Definition: Reflection of light off an irregular, dull surface.

  • Characteristics:

    • When incident rays strike an irregular surface, they reflect at varying angles, leading to scattered light in multiple directions.

    • Example surfaces: Sheet of paper, water with ripples, crumpled aluminum foil (Figures 5 and 6).

Skills Focus: Diagramming and Understanding Retro-reflectors

Equipment and Materials for Drawing Activity

  • Ruler

  • Protractor

  • Paper

  • Pencil

Steps for Drawing a Retro-reflector

  1. Draw two mirrors meeting at a right angle (L-shaped) on paper.

  2. Use a ruler to draw an incident ray hitting one mirror.

  3. Calculate the angle of incidence and draw the reflected ray based on the laws of reflection.

  4. The reflected ray now acts as the incident ray on the second mirror. Draw the second reflected ray.

  5. Use a second incident ray, striking the first mirror at a different angle, and repeat the steps. Use different colors for each ray.

Reflection Questions

  • A. How did the incident ray compare with the ray emerging after two reflections in your first trial?

  • B. Did this result change in your second trial?

  • C. What is the main characteristic of a retro-reflector based on your observations?

Reflective Properties and Dyslexia

  • Observation: Individuals with dyslexia may struggle with reading print, often due to glare from white paper.

  • Solution: Using colored filters or glasses may reduce glare and improve the reading experience.

Summary of Key Concepts

  • When light is reflected off a plane mirror:

    • The angle of incidence equals the angle of reflection.

    • The incident ray, reflected ray, and normal all lie in the same plane.

Check Your Understanding

  1. Draw a diagram differentiating the incident ray, the reflected ray, and the normal.

  2. Clearly state the two laws of reflection.

  3. Compare specular and diffuse reflection, providing examples.

  4. Discuss your preferred reflection type for classroom walls and justify your choice of paint (gloss vs. matte).

  5. Solve for the angle of reflection given specific angles of incidence (e.g., for incidence of 32°, what is the reflection angle?).

  6. Relate the concepts of specular and diffuse reflection to everyday spaces like kitchens, bathrooms, and bedrooms.