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Light · G3 and O-Level Physics

G3 Physics and O-Level hub for reflection, plane mirrors, refraction, total internal reflection, optical fibres and converging lenses.

  • SEC G3 Physics 2027
  • 8 lessons
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Before you begin

Light is a transverse electromagnetic wave. The lessons move from reflection and plane mirrors to refraction and total internal reflection, then use converging lenses to form images.

Be comfortable with: wave quantities and wavefronts from general wave properties.

Learning goals
  • 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

Lessons

Work through them in order.

Reflection and refraction

  1. What Is Light?Explain how you see luminous and non-luminous objects, and state the speed of light in a vacuum.
  2. Reflection Of LightApply the laws of reflection and describe the image a plane mirror forms.
  3. Drawing Ray Diagrams For Plane MirrorsDraw plane-mirror ray diagrams to locate an image, using dashed lines for virtual rays.
  4. Refraction Of LightApply the laws of refraction and calculate refractive index from angles or from the speed of light.
  5. Total Internal ReflectionState the conditions for total internal reflection, calculate a critical angle and explain optical fibres.

Converging lenses

  1. Thin Converging LensesDescribe how a converging lens focuses light and measure its focal length safely.
  2. Ray Diagrams For Converging LensDraw ray diagrams to find the position, size and nature of the image a converging lens forms.
  3. Applications Of Converging LensExplain where a camera, projector or magnifying glass places the object, and the image each forms.

Practise and check

Recommended nextCheck what I know: Light · G3 Physics and O-Level Physics

Topic reference

Revision

Required relationships
IdeaRelationship or testExam check
Reflectioni = rBoth angles are measured from the normal.
Refractionsin i/sin r = constantApplies to a fixed pair of media and travel direction.
Refractive indexn = c/vRefractive index has no unit.
Critical anglesin c = 1/nFor light passing from the medium into air.
Total internal reflectionhigher n to lower n, with i > cState both conditions.
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.
Light facts
  • Normal: a line perpendicular to the surface at the point of incidence.
  • Plane-mirror image: virtual, upright, the same size and laterally inverted.
  • Refraction: light bends towards the normal when it enters an optically denser medium and slows down.
  • Refractive index: n = c/v shows how much light slows in a medium, with c = 3.0 × 10⁸ m s⁻¹.
  • Total internal reflection: happens when light travels from a higher to a lower refractive index and the angle of incidence is greater than the critical angle.
Converging lens images
Object position (u)Image position (v)NatureOrientationSize
At infinityAt F (focus)RealInvertedHighly diminished (point)
u > 2fBetween f and 2fRealInvertedDiminished
u = 2fAt 2fRealInvertedSame size
f < u < 2fBeyond 2fRealInvertedMagnified
u = fAt infinity (emergent rays parallel)Image at infinity (no screen image)——
u < fSame side as objectVirtualUprightMagnified
  • Principal axis: the line through the optical centre of the lens.
  • Optical centre: the point in the thin-lens model where a ray passes straight through without changing direction.
  • Principal focus (F): the point on the principal axis where rays parallel to the axis converge after the lens.
  • Focal length (f): the distance from the optical centre to the principal focus.
  • Real image: can be formed on a screen. Virtual image: cannot.
Highest-value misconceptions
  • Reflection and refraction angles are measured from the normal, not the surface.
  • When light refracts, its speed and wavelength change but its frequency does not.
  • A real lens image comes from actual ray intersections; a virtual image comes from backward extensions.
Top exam traps
  1. Lateral inversion: a plane-mirror image is flipped left to right, not upside down.
  2. TIR conditions: state both, higher to lower refractive index and i > c.
  3. Lens ray rules: a ray parallel to the axis passes through F; a ray through the optical centre goes straight on; a ray through F leaves parallel to the axis.
  4. Virtual images: when u < f the rays diverge, so extend them backwards with dashed lines.
  5. Magnifying glass: the object must be closer to the lens than the focal length.

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