Retrieval Practice

Refraction & Total Internal Reflection — AQA A-Level Physics

Q1. Define refraction.
The change in direction of a wave when it passes through a boundary between media of different optical density, caused by a change in wave speed.
Q2. State the equation for the refractive index of a material.
n = c / c_s, where c is the speed of light in a vacuum and c_s is the speed of light in the material.
Q3. What happens to frequency when a wave refracts?
  • Frequency stays the same.
  • Only speed and wavelength change.
Q4. State Snell's law.
n1 sin theta1 = n2 sin theta2, where angles are measured from the normal.
Q5. Define the critical angle.
The angle of incidence in the denser medium at which the angle of refraction is exactly 90 degrees.
Q6. State the equation for the critical angle.
sin theta_c = n2 / n1, where n1 is the denser medium and n2 is the less dense medium.
Q7. State the two conditions required for total internal reflection.
  • 1.
  • Angle of incidence > critical angle. 2.
  • Light travels from a denser medium to a less dense medium (n1 > n2).
Q8. Name the three components of a step-index optical fibre.
Core (optically dense glass/plastic), cladding (lower refractive index), outer sheath (protective covering).
Q9. State two roles of cladding in an optical fibre.
Protects the core from damage, and ensures TIR occurs by having a lower refractive index than the core.
Q10. Explain the difference between material dispersion and modal dispersion.
  • Material dispersion: different wavelengths travel at different speeds (fix: use monochromatic light).
  • Modal dispersion: different paths through the fibre have different lengths (fix: use a narrower core).
Q11. State two methods to reduce pulse broadening in optical fibres.
  • Use a monochromatic source (reduces material dispersion).
  • Use a narrow core (reduces modal dispersion).
  • Use repeaters to regenerate the signal.
Q12. What is pulse broadening?
  • The spreading out of signal pulses as they travel through an optical fibre, caused by material and/or modal dispersion.
  • It can cause pulses to merge and information to be lost.
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