Light

Overview

This lecture explains the entire chapter "Light," including the principles and numericals of reflection, refraction, mirrors, and lenses.

Basic Principles of Light

• The speed of light in vacuum/air is 3 × 10^8 m/s.
• When light hits a surface and bounces back, it is called reflection.
• There are two laws of reflection: all rays and the normal lie in the same plane; angle of incidence = angle of reflection.

Mirrors and Their Characteristics

• In a plane mirror, the image is virtual, erect, and laterally inverted.
• The size of the image is equal to the object; the distance is also equal.
• Curved mirrors are of two types—concave (cave-like) and convex (bulging).

Rules for Ray Diagrams of Mirrors

• Parallel rays pass through the focus.
• Rays passing through the center of curvature return along the same path.
• Rays passing through the focus become parallel.
• Rays hitting the pole reflect according to the laws of reflection.

Image Formation Cases of Mirrors

• Concave mirror: the size, type, and location of the image change with the object's position.
• Convex mirror: the image is always virtual, erect, and smaller.

Lenses and Their Diagrams

• Convex lens (converging): image can be real/virtual and size depends on the object's position.
• Concave lens (diverging): image is always virtual, erect, and smaller.
• Lens diagram rules: parallel ray—through focus, ray through focus—parallel, ray through optical center—straight.

Refraction of Light

• Refraction: when light passes from one medium to another and bends.
• Snell's law: sin i/sin r = μ (ratio of refractive indices).
• When going from rarer to denser medium, the ray bends toward the normal; from denser to rarer, away from the normal.

Refractive Index and Numericals

• μ = speed of light in air/speed of light in medium.
• μ has no unit; the higher the μ, the denser the medium.

Sign Convention and Formulas

• Left-side values negative, right positive, up positive, down negative.
• Mirror formula: 1/v + 1/u = 1/f ; Lens formula: 1/v - 1/u = 1/f.
• Magnification: mirror: -v/u; lens: v/u; +m virtual/erect, -m real/inverted.

Utility and Related Questions

• Concave mirror—headlights, torch, dentist; convex—rear view mirror.
• Convex lens—magnifying glass, glasses, camera; concave lens—spyhole, glasses, etc.

Key Terms & Definitions

• Reflection — light bouncing back from a surface.
• Refraction — bending of light when changing medium.
• Refractive Index (μ) — the ability of a medium to bend light.
• Focal Length (f) — distance from pole to focus.
• Magnification (m) — height of image/height of object.
• Virtual Image — an image that cannot be formed on a screen in the real world.

Action Items / Next Steps

• Homework: Comment on types of mirrors with +3 magnification and object positions.
• Next chapter: Reproduction—turn on notifications, save the class link.
• Practice main formulas and ray diagrams.