Light Reflection and Refraction

Overview

This chapter explains the basic concepts of light, focusing on the laws and phenomena of reflection and refraction, image formation by mirrors and lenses, key formulas, and their practical uses.

Nature and Behavior of Light

• Light allows us to see objects by reflecting off surfaces or passing through transparent materials.
• Light usually travels in straight lines, as shown by the sharp shadows formed by small light sources.
• Although light has both wave and particle properties, this chapter uses the straight-line (ray) model to explain optical phenomena.

Reflection of Light

• Reflection is when light bounces off a surface.
• Laws of reflection:
  • The angle of incidence equals the angle of reflection.
  • The incident ray, reflected ray, and the normal all lie in the same plane.
• Plane mirrors create virtual, erect, laterally inverted images that are the same size and distance from the mirror as the object.
• Spherical mirrors are of two types:
  • Concave (curved inward)
  • Convex (curved outward)

Spherical Mirrors: Key Terms and Formulas

• Pole (P): Center of the mirror’s surface.
• Center of Curvature (C): Center of the sphere from which the mirror is made.
• Principal Axis: Line passing through P and C.
• Principal Focus (F): Point where parallel rays meet (concave) or appear to diverge from (convex).
• Focal Length (f): Distance from P to F; Radius of Curvature (R): Distance from P to C; R = 2f.
• Image characteristics (size, nature, position) depend on the object's location.
• Mirror formula: 1/v + 1/u = 1/f
• Magnification (m): Ratio of image height to object height; m = -v/u (for mirrors).

Refraction of Light

• Refraction is the bending of light as it passes from one medium to another at an angle, due to a change in speed.
• The amount of bending depends on the optical density and refractive index of the media.
• Laws of refraction:
  • The incident ray, refracted ray, and normal all lie in the same plane.
  • The ratio sin(i)/sin(r) is constant for a given pair of media (Snell’s law).

Refractive Index

• Refractive index (n): n = speed of light in vacuum / speed in medium.
• A higher refractive index means the medium is optically denser.
• Light bends towards the normal in denser media and away from the normal in rarer media.

Lenses and Image Formation

• Convex lens (converging): Thicker in the middle; brings parallel rays to a focus.
• Concave lens (diverging): Thicker at the edges; spreads parallel rays outward.
• Principal focus: Point where parallel rays converge (convex) or appear to diverge (concave).
• Focal length: Distance from the optical center to the principal focus.
• Convex lenses form real, inverted images for objects beyond the focal point, and virtual, erect images for objects within the focal length.
• Concave lenses always form virtual, erect, and diminished images.
• Lens formula: 1/v – 1/u = 1/f
• Magnification (m): m = v/u (for lenses).

Power of a Lens

• Power (P): P = 1/f (f in meters), measured in dioptres (D).
• Power is positive for convex lenses and negative for concave lenses.
• For lenses in contact: P = P₁ + P₂ + ...

Key Terms & Definitions

• Reflection: Bouncing of light from a surface.
• Refraction: Bending of light as it passes from one medium to another.
• Principal Focus (F): Point where parallel rays converge or appear to diverge after reflection/refraction.
• Focal Length (f): Distance from the mirror/lens pole or optical center to the principal focus.
• Refractive Index (n): Ratio of speed of light in vacuum to speed in the medium.
• Magnification (m): Ratio of image height to object height.
• Power (P): Reciprocal of focal length (in dioptres).
• Concave Mirror/Lens: Inward curved mirror (converging), or lens (diverging).
• Convex Mirror/Lens: Outward curved mirror (diverging), or lens (converging).

Action Items / Next Steps

• Draw ray diagrams for image formation by mirrors and lenses for different object positions.
• Practice numerical problems using mirror and lens formulas, magnification, and power.
• Review summary tables for image characteristics with mirrors and lenses.
• Prepare answers for end-of-chapter exercises and short questions.