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7.5Chapter 7: Light

Refraction: air and glass — Alhussan Physics - Stage 8 Revision Notes & Topical Questions

5 min read 7 parts Lesson 5 of 12 2 quizzes

📚 Alhussan Physics - Stage 8 Hub / Topical Revision
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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 7Refraction 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 7Predictions and hypotheses

Prediction
  • It should say what will happen and why, using scientific knowledge.
Hypothesis
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
Conclusion a statement of what was found out, explained using scientific knowledge.

Section 3 of 7Different 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 7Total internal reflection

1Dense to less dense
  • When light goes from glass or water into air, the angle of refraction is bigger than the angle of incidence.
2Critical angle
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.
3Total internal reflection
  • When the angle of incidence is bigger than the critical angle, all the light is reflected back inside.
4Conditions
  • Light must travel from a denser to a less dense medium and the angle of incidence must exceed the critical angle.

Section 5 of 7Uses 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.

Section 6 of 7Key terms

TermMeaning
Critical angleThe angle of incidence at which the angle of refraction is 90°.
Total internal reflectionComplete reflection of light inside a denser medium when the angle exceeds the critical angle.
Optical fibreA thin glass or plastic fibre that carries light by total internal reflection.
EndoscopeAn instrument using optical fibres to see inside the body.
HypothesisScientific knowledge used to explain why a prediction should be true.
Refractive indexA measure of how much a material slows and bends light.

Section 7 of 7Exam technique

1
Thinking light speeds up when entering glass — it slows down.
2
Thinking total internal reflection can happen from air into glass — light must be going into a less dense medium.
3
Thinking a prediction only says what will happen — it should include a scientific reason.
4
Thinking light changes direction along the normal — it does not.

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