This guide covers the behavior of light, including reflection and refraction, through mirrors and lenses. Mastering the concepts and, most importantly, the ray diagrams is crucial for your O/L examination.

1. Reflection of Light

Short Notes & Key Concepts

• Reflection: The bouncing back of light when it strikes a surface.

• Laws of Reflection (Apply to ALL mirrors):

  • The incident ray, the reflected ray, and the normal at the point of incidence all lie on the same plane.

  • The angle of incidence (i) is equal to the angle of reflection (r).

• Plane Mirrors (e.g., a dressing table mirror):

  • The image formed is virtual (cannot be projected onto a screen).

  • The image is upright (the same way up as the object).

  • The image is the same size as the object.

  • The image is laterally inverted (left and right are swapped).

  • The image distance is equal to the object distance.

2. Curved (Spherical) Mirrors

• Key Terms:

  • Pole (P): The center of the mirror's surface.

  • Center of Curvature (C): The center of the sphere from which the mirror was cut.

  • Principal Axis: The straight line passing through the Pole (P) and the Center of Curvature (C).

  • Focal Point (F): The point on the principal axis where rays parallel to the axis converge (concave) or appear to diverge from (convex) after reflection.

  • Focal Length (f): The distance from the Pole (P) to the Focal Point (F). Note: f = C / 2.

3. Ray Diagrams for Curved Mirrors (CRITICAL SKILL!)

To find the image, draw at least two of these three principal rays from the top of the object:

1. A ray parallel to the principal axis reflects through the focal point (F).

2. A ray passing through the focal point (F) reflects parallel to the principal axis.

3. A ray passing through the center of curvature (C) reflects back along the same path.

4. Concave Mirror Image Formation (Summary):

• Convex Mirror Image Formation:

  • ALWAYS forms a virtual, upright, and diminished image, no matter where the object is placed.

4. Refraction of Light

• Refraction: The bending of light as it passes from one medium to another (e.g., from air to water).

• Cause: This happens because the speed of light changes when it enters a new medium.

• Key Rules:

  • When light enters a denser medium (e.g., air to glass), it bends TOWARDS the normal.

  • When light enters a rarer medium (e.g., glass to air), it bends AWAY FROM the normal.

• Total Internal Reflection (TIR):

  • Conditions for TIR:

    1. Light must be traveling from a denser medium to a rarer medium.

    2. The angle of incidence must be greater than the critical angle (c).

  • Critical Angle (c): The specific angle of incidence in the denser medium that causes the angle of refraction in the rarer medium to be exactly 90°.

  • Application: This is the principle behind how optical fibers work, allowing light to travel long distances with minimal loss.

5. Lenses

• Convex Lens (Converging): Thicker in the middle. It converges parallel rays of light to a focal point.

  • Image Formation: The rules are very similar to a concave mirror. It can form both real and virtual images.

  • As a Magnifying Glass (Simple Microscope): When the object is placed within the focal length (u < f), a convex lens produces a magnified, virtual, and upright image.

• Concave Lens (Diverging): Thinner in the middle. It causes parallel rays of light to diverge.

  • Image Formation: Like a convex mirror, it ALWAYS forms a virtual, upright, and diminished image.

Exam Tips & Tricks

1. Practice Ray Diagrams! This is the most important skill in this chapter. Use a ruler and pencil. Always include arrows on your rays to show the direction of light.

2. Know the Tables: Memorize the summary tables for image formation by a concave mirror and a convex lens. It’s a quick way to check your ray diagrams.

3. Real vs. Virtual: A real image is formed where light rays actually intersect and can be projected onto a screen. A virtual image is formed where rays appear to come from and cannot be projected.

4. Denser vs. Rarer: Remember the rule: Fast to Slow, Towards (the normal). When light slows down in a denser medium, it bends towards the normal.

5. TIR Conditions: The two conditions for Total Internal Reflection are frequently asked. Memorize them.

Important Points to Remember

• Concave Mirror ≈ Convex Lens (Both can form real or virtual images).

• Convex Mirror ≈ Concave Lens (Both ALWAYS form virtual, upright, diminished images).

Refraction is all about the change in speed of light.