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Light Reflection and Refraction Principles

Jul 14, 2025

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Overview

The lecture covers the core principles of light reflection and refraction, including behavior with mirrors and lenses, key terminology, formulas, and image formation rules.

Reflection of Light

  • Light travels in straight lines and reflects off smooth, shiny surfaces like mirrors.
  • Reflection occurs when light bounces back from a surface.
  • Plane mirrors produce virtual, laterally reversed images of the same size as the object.
  • Spherical mirrors are either concave (inward curve) or convex (outward curve).
  • Key terms: pole (P), center of curvature (C), radius of curvature (R), principal axis, principal focus (F), focal length (f), and aperture.

Image Formation by Spherical Mirrors

  • Concave mirror image rules:
    • At infinity: image at focus, point-sized, real, inverted.
    • Beyond C: image between F and C, diminished, real, inverted.
    • At C: image at C, same size, real, inverted.
    • Between C and F: image beyond C, enlarged, real, inverted.
    • At F: image at infinity, highly enlarged, real, inverted.
    • Between P and F: image behind mirror, enlarged, virtual, erect.
  • Convex mirror image rules:
    • At infinity: image at focus behind the mirror, point-sized, virtual, erect.
    • Anywhere else: between P and F behind mirror, diminished, virtual, erect.
  • Concave mirrors: used in torches, headlights, shaving, dentist mirrors.
  • Convex mirrors: used as vehicle rear-view mirrors for wide field of view.

Mirror Formula and Magnification

  • Mirror formula: 1/v + 1/u = 1/f (v = image distance, u = object distance, f = focal length).
  • Magnification: m = h'/h = -v/u (h' = image height, h = object height).
  • Positive image height: virtual image; negative: real image.

Refraction of Light

  • Refraction is the bending of light when it passes from one medium to another due to speed change.
  • Examples: bent straw in water, enlarged lemon under water.
  • Laws:
    1. The incident ray, refracted ray, and normal all lie in the same plane.
    2. Snell’s Law: (sin i)/(sin r) = constant for given media.

Refractive Index

  • Refractive index (n) measures how much light slows in a medium versus vacuum.
  • n = speed of light in vacuum / speed in medium.
  • Water: n ≈ 1.33; Diamond: n ≈ 2.42.

Spherical Lenses

  • Convex lens: converging, thicker center, positive focal length.
  • Concave lens: diverging, thinner center, negative focal length.
  • Principal axis, optical center, aperture, and focus apply.
  • Image formation by convex lens varies by object position (real/inverted except for close objects: virtual/erect).
  • Concave lens always forms virtual, erect, diminished images.

Lens Formula and Magnification

  • Lens formula: 1/v - 1/u = 1/f.
  • Magnification: m = h'/h = v/u.

Power of a Lens

  • Power (P) = 1/focal length (f, in meters); units: diopters (D).
  • Convex lens: positive power; concave lens: negative power.

Key Terms & Definitions

  • Reflection — Bouncing back of light from a surface.
  • Refraction — Bending of light when passing between different media.
  • Principal Axis — Straight line through pole and center of curvature.
  • Focal Length (f) — Distance from mirror/lens center to focus.
  • Refractive Index — Ratio of light speed in vacuum to that in a medium.
  • Magnification (m) — Ratio of image height to object height.
  • Power of Lens (P) — Ability of lens to bend light, in diopters.

Action Items / Next Steps

  • Review ray diagrams for concave/convex mirrors and lenses.
  • Practice calculations using mirror and lens formulas.
  • Memorize Snell’s Law and refractive index values for common materials.

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