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P3.11 P3.2.3 Reflection of Light

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This table provides a concise, at-a-glance reference for the precise definitions and representations of the core terms. For IGCSE students, such a summary is invaluable for quick revision and ensuring accurate recall, directly supporting exam preparation. It reinforces the distinct yet interconnected roles of these terms in understanding light reflection.

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01 — Explanation

I. Key Terms & Definitions

Term	Definition
Incident Ray	The ray of light that approaches and strikes a surface.
Reflected Ray	The ray of light that leaves the surface after reflection.
Normal	An imaginary line drawn at 90° (perpendicular) to the surface at the point of incidence.
Angle of Incidence (i)	The angle between the incident ray and the normal.
Angle of Reflection (r)	The angle between the reflected ray and the normal.

02 — Explanation

II. The Laws of Reflection

First Law: The angle of incidence is equal to the angle of reflection (i = r).
Second Law: The incident ray, the reflected ray, and the normal all lie in the same plane.

03 — Explanation

III. Types of Reflection: Specular vs. Diffuse

Feature	Specular Reflection	Diffuse Reflection
Surface	Smooth and polished (e.g., mirror, still water).	Rough and matte (e.g., paper, clothing, walls).
Reflection	Parallel incident rays reflect as parallel rays.	Parallel incident rays scatter in many directions.
Image Formation	Forms a clear, distinct image.	Does not form a clear image; the surface is visible.

04 — Explanation

IV. Plane Mirror Images

Images formed by a plane mirror have the following properties:

Virtual: Cannot be projected onto a screen. Light rays only appear to come from the image location.
Upright: The image is the same way up as the object.
Laterally Inverted: The left and right sides of the image are swapped.
Same Size: The image is the same size as the object.
Same Distance: The image is located as far behind the mirror as the object is in front of it.

05 — Explanation

V. Quantitative Skills & Problem-Solving

Core Principle: Always apply the law i = r.
Angle Measurement: Angles of incidence (i) and reflection (r) are always measured from the normal, not the mirror surface.
Key Calculation: If you are given the angle between the ray and the mirror surface (θ), find the angle of incidence by calculating i = 90° - θ.

06 — Explanation

VI. Common Pitfalls

Angle Confusion: Mistaking the angle to the surface for the angle of incidence. Always measure from the normal.
Inversion Error: Confusing lateral (left-right) inversion with vertical (upside-down) inversion. Plane mirror images are upright.
Distance Misconception: Forgetting that the image distance is equal to the object distance.

07 — Explanation

VII. Exam Tips

Draw the Normal First: In any ray diagram, your first step after drawing the surface should be to draw the normal at the point of incidence.
Use Correct Lines: Use solid lines with arrows for real light rays and dashed lines for virtual rays (i.e., tracing back to a virtual image).
Check the Given Angle: Always read the question carefully to determine if an angle is given relative to the surface or the normal.

08 — Explanation

VIII. Key Applications

Plane Mirrors: Used in everyday life for personal grooming and decoration.
Curved Mirrors:
Concave: Used in car headlights and makeup mirrors to focus light or magnify.
Convex: Used in security and car side-view mirrors for a wider field of view.
Periscopes: Use two plane mirrors at 45° to see over obstacles.
Fiber Optics: Rely on an application of reflection called Total Internal Reflection (TIR) to transmit data.
Retroreflectors: Used in safety devices like bicycle reflectors and road signs to return light directly to its source.