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3.5.3Stoichiometry

Refractive Index — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 9 of 28 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Refractive Index

3.5.3 Refractive Index
Definition
  • Refractive index is the speed of light in a vacuum divided by the speed of light in the medium, and it has no units.
Always above one
  • Light travels more slowly in any material than in a vacuum, so the refractive index is always greater than 1.
Optical density
  • Materials with a higher refractive index are optically denser, and light travels more slowly within them.
Examples
  • Glass has a refractive index of about 1.5 and diamond about 2.4, so diamond bends light more strongly.

Part 2 of 5Total Internal Reflection

1
When light travels from a denser to a less dense medium, the angle of refraction is larger than the angle of incidence.
2
At the critical angle the refracted ray travels along the boundary, so the angle of refraction is 90°.
3
Above the critical angle no light escapes, and the ray is totally internally reflected within the denser material.
4
Total internal reflection obeys the law of reflection, with the angle of incidence equal to the angle of reflection.

Part 3 of 5Calculation and Applications

Calculation and Applications
Critical angle
  • Calculated from sin c = 1 ÷ n, where n is the refractive index of the denser material.
Optical fibres
  • Light stays inside the glass core by total internal reflection, so data can travel long distances.
Endoscopes
  • Bundles of optical fibres let doctors see inside the body using light carried along flexible tubes.
Sparkle
  • Diamond has a small critical angle, so light is totally internally reflected over a wide range of angles, giving sparkle.

Part 4 of 5Worked Example

Question

  • Water has a refractive index of 1.33.
  • Calculate the critical angle for light travelling from water into air.

  1. Use sin c = 1 ÷ n = 1 ÷ 1.33 = 0.752
  2. Inverse sinec = sin⁻¹(0.752) ≈ 48.8°
  3. Round to 2 significant figures: c ≈ 49°

Answer The critical angle is about 49°.

Part 5 of 5Exam Checklist & Tips

3.5.3 – Key Points for the Exam

  • Refractive index The ratio of the speed of light in a vacuum to its speed in a medium.
  • Critical angle The angle of incidence in the denser medium at which the angle of refraction is 90°.
  • Total internal reflection Reflection of all the light back into a denser medium when the angle of incidence exceeds the critical angle.
  • Optically denser Having a higher refractive index, so that light travels more slowly in the material.
  • Optical fibre A thin transparent fibre that guides light using total internal reflection.

Common Mistakes

  • Using sin c = n instead of sin c = 1 ÷ n.
  • Giving a refractive index smaller than 1, which is impossible for a material.
  • Expecting total internal reflection when light travels from a less dense to a denser material.
  • Confusing the critical angle, which is an angle of incidence, with the angle of reflection.

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