LightCopy

Cheat Sheet: Light – Reflection, Refraction, Lenses, and Dispersion (O Level / IGCSE Physics)

3.2.1 Reflection of Light

1. Key Terms

• Normal: A line perpendicular to the surface at the point of incidence
• Angle of Incidence (i): Angle between the incident ray and the normal
• Angle of Reflection (r): Angle between the reflected ray and the normal

2. Experiment to Demonstrate the Law of Reflection

• Use a ray box and a plane mirror placed on paper
• Shine light at different angles
• Mark incident ray, reflected ray, and normal
• Measure ∠i and ∠r with protractor → they will be equal

3. Image in a Plane Mirror

• Place object in front of a plane mirror
• Use rays from top and bottom of object
• Extend reflected rays behind mirror (dashed lines)
• Image is:
  • Same size
  • Same distance behind mirror
  • Virtual and laterally inverted

4. Law of Reflection

• Angle of incidence = angle of reflection
• Used in drawing reflected rays and ray diagrams

3.2.2 Refraction of Light

1. Key Terms

• Angle of Incidence (i): Between incident ray and normal
• Angle of Refraction (r): Between refracted ray and normal

2. Refractive Index

• n = sin i / sin r
  where:
  i = angle of incidence
  r = angle of refraction
  n = refractive index (no units)

3. Experiment to Show Refraction

• Use a glass or Perspex block and ray box
• Shine ray at an angle into the block
• Trace the incident ray, refracted ray, and emergent ray
• Measure ∠i and ∠r to calculate n

4. Total Internal Reflection (TIR)

• Critical Angle (c):
  Angle of incidence inside a dense medium for which angle of refraction = 90°
• TIR occurs when:
  • Light travels from denser to less dense medium
  • i > c
• Equation:
  n = 1 / sin c

5. Experiments for TIR

• Use semi-circular glass block
• Shine ray toward curved edge
• Increase ∠i until light reflects entirely within block (no refracted ray)

6. Optical Fibres

• Use total internal reflection to carry signals
• Telecommunications:
  • Fast data transfer
  • Less signal loss
• Medical use (endoscopy):
  • Non-invasive viewing inside body

3.2.3 Thin Lenses

1. Action of Lenses

• Converging Lens (convex):
  Rays bend inward to a point (focal point)
• Diverging Lens (concave):
  Rays spread out (appear to diverge from a focal point)

2. Key Terms

• Principal Axis: Central line through lens
• Focal Length (f): Distance from lens to principal focus
• Principal Focus (F): Point where rays converge (real) or appear to diverge from (virtual)

3. Ray Diagrams: Converging Lens

• Real Image (object beyond 2f):
  • Formed on opposite side
  • Inverted, smaller or same size
• Virtual Image (object inside f):
  • Formed on same side
  • Upright, magnified
• Real image = formed by converging rays
• Virtual image = formed by diverging rays

4. Linear Magnification

• Magnification = image height / object height
  (No unit)

5. Magnifying Glass Use

• Object placed between lens and focal point
• Produces a virtual, upright, magnified image

6. Ray Diagrams for the Eye

• Normal Eye: Image forms on retina
• Short-sighted: Image forms in front of retina
• Long-sighted: Image forms behind retina

7. Corrective Lenses

• Short-sighted (Myopia): Use diverging lens
• Long-sighted (Hyperopia): Use converging lens

3.2.4 Dispersion of Light

1. Dispersion by Prism

• White light splits into different colours when refracted by a glass prism
• Different colours refract at different angles

2. Visible Spectrum (Traditional Order)

Mnemonic: ROYGBIV – Red, Orange, Yellow, Green, Blue, Indigo, Violet