Light Reflection and Refraction

Overview

This lecture covers the complete chapter on Light, including reflection, refraction, mirrors, lenses, formulas, ray diagrams, and important numericals, along with exam-focused tricks and key definitions.

Nature of Light

• Light is a form of energy and is not visible itself but makes objects visible.
• The speed of light in a vacuum is 3 × 10⁸ m/s.
• Light travels in a straight line (rectilinear propagation) and can travel in a vacuum.

Reflection of Light

• Reflection is the bouncing back of light when it strikes a surface.
• Key terms: incident ray (incoming), reflected ray (outgoing), and normal (perpendicular at the point of incidence).
• Laws of reflection: (1) Angle of incidence = angle of reflection. (2) Incident ray, reflected ray, and normal lie in the same plane.
• Lateral inversion: Right and left appear reversed in a plane mirror.

Plane Mirror

• Images are always virtual (not real), erect (upright), laterally inverted, and of the same size as the object.
• The image distance from the mirror equals the object distance.
• The focal length of a plane mirror is infinite.

Spherical Mirrors

• Two types: concave (cave-shaped, converging) and convex (bulging outward, diverging).
• Important parts: pole (midpoint), center of curvature (center of sphere), principal axis (line joining pole and center), focus (midpoint between pole and center of curvature), focal length, and aperture (diameter).
• Relationship: Radius of curvature = 2 × focal length.

Image Formation by Concave & Convex Mirrors

• Six standard cases based on object position (e.g., at infinity, beyond C, at C, between C & F, at F, between F & P).
• Images can be real/inverted or virtual/erect; size and location depend on object position.
• PK Trick: Numbering positions helps recall image locations.

Uses of Mirrors

• Concave mirrors: shaving, make-up, dentists, headlights, solar furnace.
• Convex mirrors: vehicle rear-view mirrors, security cameras; always form virtual, diminished (smaller) images.

Sign Conventions & Mirror Formula

• Left of pole: negative; right: positive; above axis: positive; below: negative.
• Mirror formula: 1/v + 1/u = 1/f where v = image distance, u = object distance, f = focal length.
• Magnification: m = height of image/height of object = -v/u.

Refraction of Light

• Refraction: Bending of light when it passes from one medium to another due to change in speed.
• Laws: (1) Incident ray, refracted ray, and normal lie in the same plane. (2) Snell’s Law: sin i/sin r = constant.
• Refractive index (n): n = speed of light in air (c) / speed in medium (v); higher n means more bending and denser medium.

Relative Refractive Index & Glass Slab

• Relative refractive index: n₂₁ = n₂ / n₁ = v₁ / v₂.
• Refraction through glass slab causes lateral displacement (emergent ray parallel to incident ray but shifted).

Lenses: Convex & Concave

• Convex lens: converging; Concave lens: diverging.
• Key parts: optical center (O), principal axis, two foci (F₁, F₂), and their doubles (2F₁, 2F₂).
• Ray diagrams: parallel rays through focus; rays through focus become parallel; rays through center go straight.

Image Formation by Lenses

• Convex lens: six standard cases similar to concave mirror, with opposite side for real/virtual images.
• Concave lens: always forms virtual, erect, diminished images.

Lens Formula & Power

• Lens formula: 1/v – 1/u = 1/f; sign conventions similar to mirrors.
• Magnification: m = height of image/height of object = v/u.
• Power of lens: P = 1/f (f in meters); SI unit is dioptre (D).
• Convex lens: positive power; Concave lens: negative power.

Key Terms & Definitions

• Incident Ray — Incoming light ray striking a surface.
• Reflected Ray — Outgoing ray after bouncing off a surface.
• Normal — Imaginary line perpendicular to surface at point of incidence.
• Lateral Inversion — Reversal of left and right in a mirror image.
• Real Image — Formed where rays actually meet; inverted.
• Virtual Image — Formed where rays only appear to meet; upright.
• Focal Length (f) — Distance from pole to focus.
• Refractive Index (n) — Ratio indicating how much light bends in a medium.
• Power of Lens (P) — Ability to converge or diverge light, P = 1/f (in meters).
• Dioptre (D) — SI unit of lens power.

Action Items / Next Steps

• Practice numerical problems using sign conventions on mirrors and lenses.
• Draw and memorize standard ray diagrams for mirrors and lenses.
• Review/refine key formulas and definitions.
• Complete assigned homework problems from the lecture.