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Light

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

  • SEC G3 Physics 2027
  • 8 lessons
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Before you begin

Light travels in straight lines until it meets a surface, where it is reflected or refracted. The lessons start with the ray model and the law of reflection, then explain refraction, total internal reflection and optical fibres, and finish with how a converging lens forms images.

Be comfortable with: wave speed, wavelength and frequency from general wave properties, because refraction is caused by a change in the speed of light.

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
Syllabus statements covered
  • recall and use the terms normal, angle of incidence and angle of reflection to describe the reflection of light
  • state that, for reflection, the angle of incidence is equal to the angle of reflection and use this principle in constructions, measurements and calculations
  • recall and use the terms normal, angle of incidence and angle of refraction to describe the refraction of light
  • recall and apply the relationship sin i / sin r = constant to new situations or to solve related problems
  • define refractive index of a medium in terms of the ratio of speed of light in vacuum and in the medium
  • explain the terms critical angle and total internal reflection
  • apply total internal reflection to the use of optical fibres in telecommunication and medicine, stating the advantages of such use
  • describe the action of a thin converging lens on a beam of light
  • define the focal length for a converging lens
  • draw ray diagrams to illustrate the formation of real and virtual images of an object by a thin converging lens
  • describe the characteristics of images (e.g. real/virtual, magnified/diminished, and upright/inverted) formed by a thin converging lens.

Lessons

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Topic reference

Quick reference

IdeaEquation or ruleNote
law of reflectioni = rboth angles measured from the normal
law of refraction(sin i)/(sin r) = constantfor light passing between two given media
refractive indexn = (sin i)/(sin r) = c/vno unit; c = 3.0 × 10⁸ m s⁻¹
critical anglesin c = 1/nfor light going from the medium into air
total internal reflectionhigher n to lower n, and i > cboth conditions are needed
converging-lens imagesobject beyond F: real and inverted; object inside F: virtual, upright and magnifiedobject at 2F gives a same-size image at 2F
Refraction, critical angle and total internal reflection at a glass–air boundaryFor glass of index 1.50 into air of index 1.00, incidence 30 degrees gives refraction 48.6 degrees and a partially reflected ray. At the critical angle 41.8 degrees, the refracted ray is along the boundary. At incidence 50 degrees, only a reflected ray remains. Reflection angles equal incidence angles.A: Below the critical anglei = 30.0°air: n = 1.00glass: n = 1.50ir = 48.6°Refraction and partial reflectionB: At the critical anglei = 41.8°air: n = 1.00glass: n = 1.50ir = 90.0°Refracted ray along boundaryC: Above the critical anglei = 50.0° > cair: n = 1.00glass: n = 1.50iTotal internal reflection
Refraction, critical angle and total internal reflection at a glass–air boundaryFor glass of index 1.50 into air of index 1.00, incidence 30 degrees gives refraction 48.6 degrees and a partially reflected ray. At the critical angle 41.8 degrees, the refracted ray is along the boundary. At incidence 50 degrees, only a reflected ray remains. Reflection angles equal incidence angles.A: Below the critical anglei = 30.0°air: n = 1.00glass: n = 1.50ir = 48.6°Refraction and partial reflectionB: At the critical anglei = 41.8°air: n = 1.00glass: n = 1.50ir = 90.0°Refracted ray along boundaryC: Above the critical anglei = 50.0° > cair: n = 1.00glass: n = 1.50iTotal internal reflection
Glass–air boundary: at c ≈ 41.8°, the refracted ray follows the boundary. Above c, total internal reflection occurs. Arrow thickness does not indicate intensity.

For this glass–air boundary, the panels compare incidence angles of 30°, the critical angle c ≈ 41.8°, and 50°. At c, the refraction angle is 90°; total internal reflection requires i > c.

Core knowledge to remember

  • The normal is a line at 90° to a surface. The angles of incidence, reflection and refraction are all measured from it.
  • Laws of reflection: the incident ray, reflected ray and normal lie in the same plane, and i = r.
  • A plane-mirror image is virtual, upright, the same size, laterally inverted, and as far behind the mirror as the object is in front.
  • Refraction happens because light changes speed. Entering a medium where it is slower, light bends towards the normal.
  • The refractive index of a medium is the speed of light in a vacuum divided by the speed of light in the medium.
  • The critical angle is the angle of incidence in the denser medium for which the angle of refraction is 90°. Total internal reflection happens when light in the denser medium meets the boundary at more than the critical angle.
  • Optical fibres use total internal reflection to carry light in telecommunications and in medical endoscopes.
  • A converging lens brings rays parallel to its principal axis together at the principal focus. The focal length is the distance from the optical centre to the principal focus.

Common mistakes

  1. Measuring angles from the surface instead of from the normal.
  2. Dividing angles instead of their sines in n = sin i/sin r.
  3. Writing v = cn instead of v = c/n.
  4. Giving only one of the two conditions for total internal reflection.
  5. Saying the frequency of light changes when it is refracted. Only the speed and wavelength change.
  6. Drawing the two rays in a lens diagram from different points on the object.
  7. Saying a virtual image can be caught on a screen.