Explain how a laser produces a beam of light that is different from the light produced by an ordinary light bulb.

Eduqas A-Level Physics — 2.8 Lasers · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Lasers are used in many everyday applications, from barcode scanners to medical equipment. The light they produce has special properties that make it very different from the light emitted by an ordinary light bulb.

Model answer (5 marks)

A laser produces light that is coherent, monochromatic and highly collimated. In a laser, atoms are excited to a higher energy level and then stimulated to drop to a lower level, emitting photons that are all in phase (coherent) and of the same wavelength (monochromatic). The optical cavity ensures the photons travel in a narrow, parallel beam that does not spread out. In contrast, an ordinary light bulb emits incoherent light of many wavelengths, which spreads in all directions and is not collimated.

Examiner tips

  • Use the terms coherent, monochromatic and collimated; link each to the stimulated emission process.
  • Show the contrast with a bulb: many wavelengths, incoherent, divergent.
  • Keep the answer concise and directly address each point.

Mark scheme (5 marks)

  1. Laser light is coherent / all waves are in phase with each other
  2. Laser light is monochromatic / all one wavelength or colour / single frequency
  3. Laser light is a narrow, parallel / collimated beam that does not spread out
  4. In a laser, atoms are stimulated to emit photons / light energy is released when excited atoms drop to a lower energy level
  5. Ordinary light bulb produces light of many wavelengths / incoherent light / light spreads in all directions, whereas laser light does not

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