Overview
This lecture covers the chapter "Light," focusing on the concepts of reflection, refraction, mirrors, and lenses, with key definitions, diagrams, formulas, and exam-oriented questions.
Nature of Light
- Light is a form of energy and enables vision.
- Light travels in a straight line (rectilinear propagation) at a speed of 3 × 10⁸ m/s.
- Light can travel through vacuum.
Reflection of Light
- Reflection is the bouncing back of light from a surface.
- Incident ray (incoming), reflected ray (outgoing), and normal (perpendicular to surface) are key rays.
- Law 1: Angle of incidence = Angle of reflection (i = r).
- Law 2: Incident ray, reflected ray, and normal all lie in the same plane.
- Lateral inversion: right and left reversal in a plane mirror (e.g., ambulance sign).
Plane Mirror Properties
- Forms virtual, erect, and laterally inverted images.
- Image size equals object size.
- Image distance from mirror = object distance from mirror.
- Focal length of a plane mirror is infinite.
Spherical Mirrors: Types & Definitions
- Two types: Concave (converging, cave-shaped), Convex (diverging, bulging outward).
- Pole: Midpoint of mirror’s surface.
- Centre of curvature (C): Center of the sphere of which the mirror is a part.
- Principal axis: Line passing through pole and center of curvature.
- Principal focus (F): Point where parallel rays converge/diverge.
- Focal length (f): Distance from pole to focus.
- Radius of curvature (R): Distance from pole to center of curvature (R = 2f).
- Aperture: Diameter of the mirror’s reflecting surface.
Image Formation by Spherical Mirrors
- Ray rules for construction:
- Parallel to axis → passes through (or appears from) focus.
- Through focus → goes parallel.
- Through center of curvature → returns on same path.
- Incident at pole → reflects at equal angle.
- For concave mirrors: image properties depend on object position (infinity, beyond C, at C, between C and F, at F, between F and pole).
- For convex mirrors: always forms virtual, erect, diminished images between pole and focus.
Uses of Spherical Mirrors
- Concave: Shaving/makeup mirrors, dentist’s mirror, solar furnace, headlights.
- Convex: Rear view mirrors, security mirrors.
Sign Conventions
- Distances measured left of pole/optical center are negative; right are positive.
- Heights above axis are positive; below are negative.
- For mirrors: object distance (u) is always negative, focal length negative for concave, positive for convex.
Mirror Formula & Magnification
- Mirror formula: 1/v + 1/u = 1/f.
- Magnification (m): height of image / height of object = -v/u.
- Negative magnification: real, inverted image; positive: virtual, erect image.
Refraction of Light
- Refraction: bending of light as it passes from one medium to another due to speed change.
- Laws of refraction:
- Incident ray, refracted ray, and normal are in the same plane.
- Snell’s Law: sin i / sin r = constant = refractive index (n).
- Ray bends toward normal from rarer to denser medium; away for denser to rarer.
- Refractive index formula: n = speed of light in air / speed of light in medium.
Lenses: Types, Rules & Image Formation
- Two types: Convex (converging), Concave (diverging).
- Convex lens brings parallel rays to focus; concave spreads rays.
- Principal axis, optical center, focal points (F1 and F2), and 2F points are key features.
- Rules for ray diagram:
- Parallel to axis → through focus (convex) or appears from focus (concave).
- Through focus → emerges parallel.
- Through optical center → passes straight.
- Lens formula: 1/v - 1/u = 1/f.
- Magnification: height of image / height of object = v/u.
- Sign convention same as mirrors; convex lens f is positive, concave is negative.
Power of Lens
- Power (P): ability to converge or diverge light; P = 1/f (in meters), or P = 100/f (in cm).
- SI unit: Dioptre (D).
- Convex lens: positive power; concave lens: negative power.
Uses of Lenses
- Convex: Magnifying glass, spectacles for hypermetropia, microscopes, telescopes.
- Concave: Spectacles for myopia, peepholes, beam expanders.
Key Terms & Definitions
- Rectilinear propagation — Light travels in straight lines.
- Reflection — Bouncing back of light from a surface.
- Refraction — Bending of light when it enters a new medium.
- Incident ray — Ray striking the surface.
- Reflected ray — Ray bouncing away from surface.
- Normal — Imaginary line perpendicular to surface.
- Lateral inversion — Right/left reversal in mirror images.
- Pole (P) — Center of mirror surface.
- Centre of curvature (C) — Center of sphere of mirror.
- Principal focus (F) — Point where parallel rays converge/diverge.
- Focal length (f) — Distance from pole/optical center to focus.
- Aperture — Diameter of reflecting/refracting surface.
- Refractive index (n) — Measure of how much a medium bends light.
- Power of lens — 1/f (m) or 100/f (cm), in dioptres.
Action Items / Next Steps
- Review and practice ray diagrams for concave/convex mirrors and lenses.
- Memorize mirror and lens formulas and sign conventions.
- Complete assigned numerical and conceptual homework problems on reflection and refraction.
- Read next NCERT section on human eye and optical instruments.