Reflection & Refraction Of Light

Key idea: G3 Physics and O-Level light hub: reflection, refraction, mirror ray diagrams, total internal reflection and optical fibres.

  • SEC G3 Physics 2027
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Learning objectives

  • Describe wave generation by vibrating sources, ropes and springs
  • Describe ripple-tank waves using wavefronts
  • Explain that waves transfer energy
  • Explain that wave energy transfer does not transfer matter
  • Use amplitude, frequency and wavelength to describe wave motion
  • Define and use wave speed and period and interpret wave graphs
  • Recall and apply wave speed = frequency × wavelength
  • Compare transverse and longitudinal waves and give examples
  • Explain sound production by vibration and the need for a medium
  • Describe sound using compressions and rarefactions
  • Relate sound loudness to amplitude and pitch to frequency
  • Explain reflected-sound echoes and use them to measure distance
  • Explain ultrasound use in sonar and soft-tissue scanning
  • Use the normal, angle of incidence and angle of reflection
  • Apply the law of reflection in constructions, measurements and calculations
  • Use the normal, angle of incidence and angle of refraction
  • Apply sin i divided by sin r as a constant for a fixed pair of media
  • Define refractive index as vacuum light speed divided by medium light speed
  • Explain the critical angle
  • Explain the conditions for total internal reflection
  • Apply total internal reflection to optical fibres and state advantages
  • Describe how a thin converging lens acts on a light beam
  • Define the focal length of a converging lens
  • Construct real and virtual image ray diagrams for a thin converging lens
  • Describe lens images as real or virtual, magnified or diminished, and upright or inverted

Light is a transverse electromagnetic wave. Use this hub to build the ideas in order: reflection, refraction, refractive index, total internal reflection and optical fibres. Continue to converging lenses after this sequence.

Waves Overview

Light is a major sub-topic of the Waves Hub.

Start here

Before you begin:

Follow this order:

  1. What is Light?
  2. Reflection & Mirror Diagrams
  3. Refraction & Snell’s Law
  4. Total Internal Reflection (TIR)

Visual checkpoint: use the Light & Lens Explorer after step 4 and explain every angle from the normal.

Lessons

Reflection

Refraction & TIR

  • Refraction

    Bending of light and refractive index calculations.

  • Total Internal Reflection

    Conditions for TIR and optical fiber applications.

Revision

Refraction, critical angle, and total internal reflectionThree ray diagrams show normal incidence into a higher-index medium, the critical-angle case from higher to lower index, and total internal reflection beyond the critical angle.A: Normal incidenceair, lower nglass, higher ndirection unchangedB: At i = clower nhigher ncr = 90°C: At i > clower nhigher nitotal internal reflection
At normal incidence the ray does not bend, although its speed and wavelength still change. From higher to lower refractive index, the refracted ray reaches the boundary at i = c; total internal reflection occurs only for i > c.
Quick Reference
ConceptFormula / LawKey Detail
Reflectioni = rIncidence = Reflection
Refractionn = (sin i)/(sin r)Snell’s Law (Air to Medium)
Wave Speedn = c/vc = 3 × 10⁸ m s⁻¹
Critical Anglen = 1/(sin c)For a boundary from the medium to air
Quick facts to remember
  • Normal: An imaginary line perpendicular (90°) to the surface at the point of incidence.
  • Refractive Index (n): A ratio showing how much light slows down in a medium.
  • Total Internal Reflection: Occurs when light travels from a denser to a less dense medium and the angle of incidence is greater than the critical angle.
  • Image in Plane Mirror: Virtual, upright, same size, and laterally inverted.
  • Bending: Light bends towards the normal when entering a denser medium (slowing down).
Top Exam Traps
  1. Angles: Always measure angles (incidence, reflection, refraction) from the NORMAL, not the surface.
  2. Lateral Inversion: In a mirror, the image is flipped left-to-right (Laterally Inverted), not upside down.
  3. Frequency: When light refracts, its speed and wavelength change, but its frequency stays constant.
  4. TIR Conditions: TIR can happen only when light travels from a higher refractive index to a lower refractive index, with i > c.
  5. Virtual Image: You cannot capture a virtual image on a screen because the light rays do not actually meet there.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027