In-depth notes on Light – Reflection and Refraction

Science: Light – Reflection and Refraction

Key Concepts

• Visibility: Objects become visible when light reflects off them, which is received by our eyes.
• Propagation of Light: Light generally travels in straight lines, as evidenced by shadows cast by opaque objects.
• Diffraction: Light can bend around small objects, demonstrating wave-like properties when treated as a wave.
• Quantum Theory: Light exhibits both wave and particle properties, explained by modern quantum theory.

9.1 Reflection of Light

• Laws of Reflection:
  • Law 1: Angle of incidence (i) = Angle of reflection (r).
  • Law 2: Incident ray, normal, and reflected ray lie in the same plane.
• Image Formation by Plane Mirrors:
  • Image is virtual and erect.
  • Size of the image equals the size of the object.
  • Image is laterally inverted.

9.2 Spherical Mirrors

• Types of Spherical Mirrors:
  • Concave Mirror: Curved inwards, converges rays.
  • Convex Mirror: Curved outwards, diverges rays.
• Key Terms:
  • Pole (P): Center of the reflecting surface.
  • Centre of Curvature (C): Center of the sphere from which mirror is derived.
  • Radius of Curvature (R): Distance between pole and center of curvature.
  • Principal Axis: Line through the pole and center of curvature.
  • Focus (F): Where parallel rays converge (concave) or appear to diverge (convex).
• Image Formation by Spherical Mirrors:
  • The nature and size of the image depend on object position concerning points P, F, and C.
• Ray Diagrams:
  • Used to illustrate image formation by concave and convex mirrors.
  • Key rays include one parallel to the principal axis, one through the focus, and one through the center of curvature.

9.2.1 Image Characteristics**

• Concave Mirror
  • Object at Infinity: Image at F (highly diminished, real and inverted).
  • Object beyond C: Image between F and C (diminished, real and inverted).
  • Object at C: Image at C (same size, real and inverted).
  • Object between C and F: Image beyond C (enlarged, real and inverted).
  • Object at F: No image formed.
  • Object between P and F: Image behind the mirror (enlarged, virtual and erect).
• The relationship between R and f is R = 2f.

9.2.4 Mirror Formula and Magnification

• Mirror Formula:
  \frac{1}{v} + \frac{1}{u} = \frac{1}{f}
• Magnification (m):
• m = \frac{h'}{h} = -\frac{v}{u}
• Real images: negative, virtual images: positive.

9.3 Refraction of Light

• Phenomenon: Light bends when entering a different medium (i.e., air to water).
• Laws of Refraction:
  • Incident ray, refracted ray, and normal are coplanar.
  • Snell's Law: \frac{\sin i}{\sin r} = n (index of refraction).
• Refraction Index: Describes how light propagates through different media. High index = slower light speed.

9.3.3 Refraction by Spherical Lenses

• Types of Lenses:
  • Convex Lens: Converges light, thickest in center.
  • Concave Lens: Diverges light, thickest at edges.
• Image Formation by Lenses: Similar to mirrors but obeys lens-specific formula.

9.3.7 Lens Formula and Magnification

• Lens Formula: \frac{1}{v} - \frac{1}{u} = \frac{1}{f}
• Lens Magnification: m = \frac{h'}{h} = \frac{v}{u}.

9.3.8 Power of a Lens

• Power (P):
• P = \frac{1}{f} (in metres), unit: dioptres (D)
• A positive power indicates a convex lens; a negative indicates a concave lens.

Conclusion

• Understanding these properties of light and their practical applications (in optics, lenses, mirrors) is essential in physics.
• Mastering images formed by mirrors and lenses elucidates the behavior of light through reflection and refraction.