Overview
This lecture covers the main concepts of light reflection and refraction, detailing the behavior of light with mirrors and lenses, including key definitions, formulas, and image formation rules.
Reflection of Light
- Reflection occurs when light bounces back from a smooth surface like a mirror.
- Plane mirrors form virtual images the same size as the object, located behind the mirror, and laterally reversed.
- Spherical mirrors are curved and can be convex (outward) or concave (inward).
- Key parts of a spherical mirror: pole (P), center of curvature (C), radius of curvature (R), principal axis, principal focus (F), focal length (f), and aperture.
- For spherical mirrors, R = 2f (radius of curvature = twice the focal length).
- Ray diagram rules: incident rays parallel to the principal axis reflect through or appear from F; rays through F reflect parallel; rays through C retrace their path.
- Concave mirror image formation varies by object position (at infinity, beyond C, at C, between C and F, at F, between P and F).
- Convex mirrors always produce virtual, diminished, and erect images behind the mirror.
Uses and Sign Conventions
- Concave mirrors: used in shaving mirrors, dentist mirrors, torch/vehicle reflectors.
- Convex mirrors: used as rear-view mirrors due to wide field of view.
- Sign convention: object left of mirror (negative), distances measured from pole, right (positive x), left (negative x), above (positive y), below (negative y).
Mirror Formula and Magnification
- Mirror formula: 1/v + 1/u = 1/f (v = image distance, u = object distance, f = focal length).
- Magnification (m): m = height of image (h') / height of object (h) = -v/u.
Refraction of Light
- Refraction: change of light direction when passing between different media, due to speed change.
- Light bends towards normal entering denser medium (e.g., air to water).
- Laws of refraction: 1) Incident ray, refracted ray, and normal lie in the same plane. 2) sin i / sin r = constant (Snell's Law).
- Refractive index (n): n = speed in vacuum (c) / speed in medium (v).
- Absolute refractive index: n = c/v; higher n means slower light.
Refraction by Lenses
- Spherical lenses: convex (converging) or concave (diverging).
- Key parts: centers of curvature, principal axis, optical center, aperture, principal focus (F), focal length (f).
- Convex lens: thicker in center, converges rays to F.
- Concave lens: thinner in center, diverges rays from virtual F.
- Ray diagram rules: parallel rays through lens go through (convex) or appear from (concave) F; rays through optical center undeviated.
Image Formation by Lenses
- Convex lens: image varies based on object position (at infinity, beyond 2F, at 2F, between F and 2F, at F, between O and F).
- Concave lens: always forms virtual, diminished, erect image.
- Sign convention: distances from optical center, convex f positive, concave f negative.
Lens Formula, Magnification, and Power
- Lens formula: 1/v - 1/u = 1/f.
- Magnification: m = h'/h = v/u.
- Power of lens (P): P = 1/f (in meters), unit is diopter (D), positive for convex, negative for concave.
Key Terms & Definitions
- Reflection — Bouncing back of light from a surface.
- Refraction — Bending of light as it passes from one medium to another.
- Refractive Index (n) — Ratio of speed of light in vacuum to that in a medium.
- Focal Length (f) — Distance from mirror/lens to principal focus.
- Magnification (m) — Ratio of image height to object height; for mirrors, m = -v/u; for lenses, m = v/u.
- Power of Lens (P) — Ability to bend light; P = 1/f (in diopters).
Action Items / Next Steps
- Practice drawing ray diagrams for mirrors and lenses.
- Memorize mirror and lens formulas and sign conventions.
- Review real-life uses of concave and convex mirrors and lenses.