Explain how a laser produces a beam of light that is different from the light produced by an ordinary light bulb.
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 procedures. The light they produce has special properties that make them useful.
Model answer (5 marks)
A laser produces light that is coherent, monochromatic and collimated. The light is coherent because the waves are in phase, giving a fixed phase relationship. It is monochromatic, meaning all photons have essentially the same wavelength (colour). The beam is collimated, so it remains narrow and does not spread as it travels. In contrast, an ordinary light bulb emits incoherent light: the waves are not in phase and the light is not coherent. The bulb also emits a broad spectrum of wavelengths, so it is not monochromatic. Finally, bulb light is emitted in all directions and the beam spreads rapidly.
Examiner tips
- Use the exact terms – coherent, monochromatic, collimated – to match the mark scheme.
- Explain each property briefly and contrast with the bulb in the same sentence for clarity.
- Show the differences in one or two short sentences to keep the answer concise.
Common mistakes
- Confusing coherence with intensity or brightness.
- Forgetting to mention that a bulb emits many wavelengths (not monochromatic).
- Describing the bulb as ‘diffuse’ but not explicitly stating it is incoherent.
Mark scheme (5 marks)
- Laser light is coherent / all waves are in phase with each other
- Laser light is monochromatic / all one wavelength (or frequency / colour)
- Laser light is collimated / the beam does not spread out (unlike an ordinary bulb)
- An ordinary light bulb emits light of many different wavelengths / is not monochromatic
- An ordinary light bulb emits light in all directions / waves are not in phase / is not coherent
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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