Describe the properties of laser light that make it different from the light produced by a torch, and explain why these properties make lasers suitable for reading data from a compact disc (CD).
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Lasers are used in many everyday devices, including CD players, barcode scanners and medical equipment. The light they produce has special properties that make them useful for such precise tasks.
Model answer (5 marks)
Laser light is monochromatic – it consists of a single wavelength, giving a pure, single colour. It is coherent – all the waves are in phase, so the light maintains a fixed phase relationship. It is highly collimated – the beam is narrow and parallel, so it does not spread out over distance.
These properties make lasers ideal for reading a CD. The narrow, focused beam can be aimed precisely onto the tiny pits and lands on the disc surface. Because the light is coherent and monochromatic, the reflected light produces a clear, detectable pattern of interference, which the CD reader decodes into data.
These properties make lasers ideal for reading a CD. The narrow, focused beam can be aimed precisely onto the tiny pits and lands on the disc surface. Because the light is coherent and monochromatic, the reflected light produces a clear, detectable pattern of interference, which the CD reader decodes into data.
Examiner tips
- Use the exact terms – monochromatic, coherent, collimated. Show how each property links to CD reading. Keep answer concise and to the point.
Common mistakes
- Confusing laser light with any bright light; failing to mention all three properties. Mixing up coherence with intensity or beam width.
Mark scheme (5 marks)
- Laser light is monochromatic (single colour / single wavelength / single frequency)
- Laser light is coherent (waves are in phase / all waves have the same phase relationship)
- Laser light is collimated / forms a narrow parallel beam that does not spread out
- The narrow / focused beam can be directed precisely onto the small pits and lands on the surface of the CD
- The reflected light is detected and the pattern of reflections is used to decode / read the data stored on the CD
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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