A student says: 'Laser light and the light from a torch are both forms of electromagnetic radiation, so they must behave in the same way.' Evaluate this statement, referring to the properties of laser light.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Both are electromagnetic radiation, so they are both light.
Laser light is monochromatic – it contains essentially one wavelength, whereas torch light is polychromatic.
Laser light is coherent – the waves are in phase, giving a fixed phase relationship; torch light is incoherent.
Laser light is highly collimated – it forms a narrow, parallel beam that spreads very little; torch light diverges widely.
Hence the statement is incorrect – the distinct properties of laser light mean it does not behave the same way as torch light.
Laser light is monochromatic – it contains essentially one wavelength, whereas torch light is polychromatic.
Laser light is coherent – the waves are in phase, giving a fixed phase relationship; torch light is incoherent.
Laser light is highly collimated – it forms a narrow, parallel beam that spreads very little; torch light diverges widely.
Hence the statement is incorrect – the distinct properties of laser light mean it does not behave the same way as torch light.
Examiner tips
- State the commonality first, then list the key differences in order of importance.
- Use the exact terms: monochromatic, coherent, collimated.
- Show the conclusion clearly that the statement is wrong.
- Keep each point brief but complete.
Mark scheme (5 marks)
- The student is correct that both are forms of electromagnetic radiation / both are light
- Laser light is monochromatic — it consists of a single wavelength / frequency (whereas torch light contains many wavelengths)
- Laser light is coherent — the waves are in phase with each other (whereas torch light is incoherent)
- Laser light is highly collimated — it forms a narrow, parallel beam that does not spread out (whereas torch light spreads out in all directions)
- The statement is therefore incorrect / partially correct because the properties of laser light are distinctly different from torch light, so they do not behave in the same way
Key terms in this question
Related
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