Signals are sent along a glass optical fibre as pulses of infrared.
The speed of infrared in the glass is 2.0 × 108 m/s.
The speed of infrared in a vacuum is 3.0 × 108 m/s.
The fibre is 12 km long.
Calculate the time taken for a pulse to travel along the fibre.
Calculate how much longer the pulse takes to travel 12 km in the glass than it would take to travel 12 km in a vacuum.
Each pulse contains a small range of wavelengths. In glass, longer wavelengths of infrared travel slightly faster than shorter wavelengths.
Explain why each pulse becomes longer (more spread out) as it travels along the fibre.
Suggest why using a source that emits a narrower range of wavelengths reduces this effect.
84 exam-style questions on Edexcel GCSE Physics 5.2 The electromagnetic spectrum, covering 5.2.1 Electromagnetic waves and refraction in glass (core practical), 5.2.2 Groups of the electromagnetic spectrum, 5.2.3 Wavelength-dependent behaviour of EM waves, and 5.2.4 Velocities of EM waves in different substances. Each one has a worked solution and a mark scheme showing where the marks go.