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Refraction, total internal reflection and optical fibresAQA A-Level Physics: Flashcards

What these 13 flashcards ask

  • Define the refractive index of a substance.
  • What is the refractive index of air, approximately?
  • State Snell's law.
  • Light passes into a medium of higher refractive index. What happens to its speed and direction?
  • Define the critical angle.
  • Give the equation for the critical angle.
  • State the two conditions for total internal reflection.
  • Why must the cladding have a lower refractive index than the core?
  • Give two other functions of the cladding.
  • What is modal dispersion?
  • What is material dispersion?
  • Why is pulse broadening a problem for a digital signal?
  • What effect does absorption have on a signal in a fibre?

Exam questions on Refraction, total internal reflection and optical fibres

  1. A narrow beam from a laser pointer travels through air and strikes the flat surface of a rectangular glass block. The glass has a refractive index of 1.50. Take the speed of light in air as 3.00 × 10⁸ m s⁻¹ and the refractive index of air as 1.00.
    The laser beam is later directed from inside the glass towards the glass–air surface at an angle of incidence of 50°. Show, by calculation, what happens to the light at this surface.2 marks
  2. A tank contains a layer of water of refractive index 1.33 with a layer of oil of refractive index 1.47 floating on top, separated by a flat horizontal boundary. A ray of light travels upwards through the water and meets this boundary at an angle of incidence of 30°.
    Calculate the speed of light in the oil, and explain why it is less than the speed of light in the water.2 marks
  3. A step-index optical fibre has a core of refractive index 1.48 surrounded by cladding of refractive index 1.46. A laser sends pulses of light along the fibre to carry digital data.
    Explain why light entering the core at a small angle to the axis stays inside the core for the whole length of the fibre.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).