Overview
This lecture explains the concepts of reflection and refraction of light, focusing on spherical mirrors and lenses, their image formation, related formulas, and applications.
Nature and Propagation of Light
- Light enables vision by reflecting from objects into our eyes.
- Light travels in a straight line, forming sharp shadows (rectilinear propagation).
- Diffraction and quantum theory reconcile wave and particle nature of light (advanced topic).
Reflection of Light
- Reflection occurs when light bounces off a polished surface like a mirror.
- Laws of reflection: (1) Angle of incidence equals angle of reflection; (2) Incident ray, normal, and reflected ray lie in the same plane.
- Plane mirrors produce virtual, erect images of equal size, laterally inverted, as far behind as the object is in front.
Spherical Mirrors
- Spherical mirrors are of two types: concave (inward curve) and convex (outward curve).
- Key terms: pole (P), centre of curvature (C), radius of curvature (R), principal axis, principal focus (F), and focal length (f).
- For small apertures, R = 2f.
Image Formation by Spherical Mirrors
- Concave mirrors can form real or virtual images depending on object position.
- Convex mirrors always form virtual, diminished, and erect images.
- Ray diagrams use two principal rays for image location.
Mirror Formula and Magnification
- Mirror formula: 1/v + 1/u = 1/f.
- Magnification (m): ratio of image height to object height, also m = v/u.
Refraction of Light
- Refraction is the bending of light when passing obliquely between different media due to speed changes.
- Laws of refraction (Snell’s Law): incident ray, refracted ray, and normal are coplanar; sin(i)/sin(r) = constant (refractive index).
- Absolute refractive index: ratio of light speed in vacuum to that in the medium.
Lenses and Image Formation
- Lenses are made of transparent materials bounded by at least one spherical surface.
- Convex lens (thicker at middle) converges light; concave lens (thicker at edges) diverges light.
- Key points: optical centre (O), principal axis, principal foci (F1, F2), and focal length (f).
- Convex lens forms real/inverted or virtual/erect images; concave lens always forms virtual, erect, diminished images.
- Lens formula: 1/v – 1/u = 1/f; Magnification: m = v/u or h'/h.
Power of a Lens
- Power (P) is reciprocal of focal length (f in meters): P = 1/f.
- SI unit is dioptre (D); convex lens power is positive, concave is negative.
- Powers of lenses in contact are additive.
Key Terms & Definitions
- Reflection — Bouncing of light from a surface.
- Refraction — Bending of light as it enters a new medium at an angle.
- Spherical mirror — Mirror with a surface that is part of a sphere (concave or convex).
- Pole (P) — Centre of the mirror's surface.
- Centre of curvature (C) — Centre of the sphere of which mirror is a part.
- Principal axis — Line passing through P and C.
- Principal focus (F) — Point where parallel rays converge/diverge.
- Focal length (f) — Distance between P and F.
- Magnification (m) — Ratio of image height to object height.
- Lens — Transparent object bounded by at least one spherical surface.
- Optical centre (O) — Central point of the lens.
- Power of lens (P) — Measure of lens’s converging/diverging ability, P = 1/f (in m⁻¹ or dioptre).
Action Items / Next Steps
- Complete textbook exercises on page 160.
- Draw ray diagrams for spherical mirrors and lenses as per the activities described.
- Practice using the mirror and lens formulas for numerical problems.
- Memorize sign conventions, formulas, and key definitions for review.