Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

PHYSICS · Stage 8 · Chapter 7: Light

Physics 7.5 Refraction: air and glass — Refractive Index and Total Internal Reflection

5 min read 5 parts Lesson 5 of 12

0%read
Lesson summary

Physics 7.5 Refraction: air and glass — Refractive Index and Total Internal Reflection — Alhussan International Academy — Physics (Stages 7–9) notes: Describe refraction through a glass block, explain different amounts of refraction and describe total internal reflection and its …

  • Air into glass
  • Glass into air
  • Emergent ray
  • Along the normal
  • Prediction
  • Hypothesis

Section 1 of 5Refraction in a glass block

  • Air into glass
    • Light slows down and bends towards the normal, so the angle of refraction is smaller than the angle of incidence.
  • Glass into air
    • Light speeds up and bends away from the normal, so the angle of refraction is bigger than the angle of incidence.
  • Emergent ray
    • The ray leaving a rectangular block is parallel to the ray entering, because the bends are equal and opposite.
  • Along the normal
    • The ray does not change direction but still slows down.

Section 2 of 5Predictions and hypotheses

  • Prediction
    • It should say what will happen and why, using scientific knowledge.
  • Hypothesis = the scientific knowledge used to explain why a prediction should be true.
    • For example, denser glass slows light more than plastic, so it refracts light more.
  • Testing
    • The prediction is tested by an experiment to collect data.
  • Conclusion = a statement of what was found out, explained using scientific knowledge.

Section 3 of 5Different materials refract differently

  • Refractive index
    • It compares the speed of light in a vacuum (or air) with its speed in the material and has no unit.
  • Slowing and bending
    • The more light slows down, the higher the refractive index and the more it bends.
  • Values
    • Diamond about 2.4, glass about 1.5, water about 1.3.
  • Density
    • Denser materials refract light more than less dense materials.

Section 4 of 5Total internal reflection

  • Dense to less dense
    • When light goes from glass or water into air, the angle of refraction is bigger than the angle of incidence.
  • Critical angle = the angle of incidence at which the angle of refraction is 90°.
    • As the angle of incidence increases, the refracted ray bends closer to the surface.
    • At the critical angle the refracted ray travels along the boundary.
  • Total internal reflection
    • When the angle of incidence is bigger than the critical angle, all the light is reflected back inside.
  • Conditions
    • Light must travel from a denser to a less dense medium and the angle of incidence must exceed the critical angle.

Section 5 of 5Uses of total internal reflection

  • Optical fibre
    • A very thin fibre of glass or plastic acts as a light pipe, with light totally internally reflected along it.
    • Optical fibres also carry telephone and internet signals.
  • Endoscope
    • It is used to see inside the human body.
    • One bundle of optical fibres sends light onto a structure inside the body.
    • Another bundle carries reflected light to a camera, so a doctor can see an image and diagnose a problem.
  • Prisms
    • Binoculars and periscopes use prisms that rely on total internal reflection.

Last updated: