A laser is used in a supermarket barcode scanner to read the barcodes on products. Explain why laser light is more suitable for this purpose than light from an ordinary light bulb.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Barcode scanners work by shining a beam of light onto the black and white lines of a barcode and detecting the reflected light. The scanner must be able to read the barcode accurately from a small spot on the barcode surface.
Model answer (5 marks)
Laser light is monochromatic, so the detector can be tuned to a single wavelength.
Laser light is coherent – all waves are in phase, giving a stable intensity.
Laser light is collimated, so it does not spread and remains a narrow beam over a distance.
The narrow beam illuminates a very small spot on the barcode, allowing the fine black and white lines to be distinguished accurately.
In contrast, light from an ordinary bulb spreads in all directions and is not collimated, so it would illuminate a large area and the barcode could not be read reliably.
Laser light is coherent – all waves are in phase, giving a stable intensity.
Laser light is collimated, so it does not spread and remains a narrow beam over a distance.
The narrow beam illuminates a very small spot on the barcode, allowing the fine black and white lines to be distinguished accurately.
In contrast, light from an ordinary bulb spreads in all directions and is not collimated, so it would illuminate a large area and the barcode could not be read reliably.
Examiner tips
- Mention monochromaticity, coherence, collimation and the small spot size in separate points.
- Use the exact terms "monochromatic", "coherent", "collimated" and "narrow beam" as the mark scheme rewards.
- Explain the disadvantage of bulb light – it spreads and is not collimated.
- Keep each point brief – 5 points total.
Mark scheme (5 marks)
- Laser light is monochromatic (a single frequency/wavelength), so the detector can be tuned to respond to exactly that wavelength
- Laser light is coherent, meaning all the waves are in phase with each other
- Laser light is collimated / does not spread out, so it remains a narrow beam over a distance
- The narrow beam means the light hits a very small spot on the barcode, allowing the fine black and white lines to be distinguished accurately
- Ordinary light bulb light spreads out in all directions / is not collimated, so it would illuminate too large an area to read the barcode reliably
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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