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Law Of Reflection & Refraction

Key idea: H3 Photonics: Law Of Reflection & Refraction — key ideas and exam-focused notes on reflection, refraction, polarisation, fibres, and attenuation.

  • Advanced Physics
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Learning objectives

  • Use advanced reference material to reinforce mathematical, optical, material, and modern-physics models.

Law Of Reflection

Reflection and refraction measured from the normalPanel A shows an incident and reflected ray making equal angles with the normal at a mirror. Panel B shows a ray entering glass from air and bending towards the normal because its speed decreases.A. Reflection: i = rnormalirplane mirrorB. Air → glass: bends towards normalnormalair: fasterglass: slowerir
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Measure every angle from the normal. Reflection gives i = r; refraction towards the normal indicates that light has entered a medium where it travels more slowly.

When light meets a boundary(from one medium to another), the forward propagation by scattering is disrupted. The incident light is now scattered backwards without cancellation, this is reflection. Law of reflection states that

  • Angle of incident is equal to angle of reflection. θi = θr
  • The incident ray, the normal to the surface and the reflected ray all lay on the same plane called the plane of incidence.

Law Of Refraction

Upon entry into the surface, lateral scattering becomes insignificant and thus forward transmission predominates. However, it is observed that when the incident light is directed an angle to the normal, the ray bends. This effect is called refraction. The bending is essentially due to the difference in speed as the beam travels from one medium to another and that the width of the beam is finite. Law of Refraction states that

  • nᵢ sin θᵢ = nₜ sin θₜ
  • The incident and refracted rays all lie in the same plane of incidence.

Total Internal Reflection

Refraction, the critical ray and total internal reflectionThree rays travel from glass towards air. A ray below the critical angle refracts away from the normal, the critical ray travels along the boundary, and a ray above the critical angle reflects back into the glass.air (lower n)glass (higher n)i < c: refractsi = c: ray along boundaryi > c: reflected
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Total internal reflection occurs only from higher to lower refractive index when the incidence angle exceeds the critical angle.

To find the critical angle: sin θ_c = nₜ/nᵢ Back To Photonics

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Course and syllabus information
Course
Advanced Physics
Edition
Advanced Physics