AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Comparison of the eye and CCD as detectors in terms of quantum efficiency,…
Full specification wording
Advantages of large diameter telescopes (A-level only): Comparison of the eye and CCD as detectors in terms of quantum efficiency, resolution, and convenience of use.
A detector records the light gathered by a telescope.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 4 of 4
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A detector records the light gathered by a telescope. The eye detects visible light through rod and cone cells, but only a small fraction of incident photons trigger a response, so its quantum efficiency is low, often a few per cent. A CCD is a silicon array that converts photons into electrons; quantum efficiency can exceed 80 per cent across a broad range of wavelengths. The eye has limited resolution set by the spacing of receptors and cannot accumulate light, whereas a CCD has small pixels that give high spatial resolution and can integrate for long periods. CCDs are also more convenient: images are recorded digitally, stored, processed and shared, while the eye gives a real-time but non-permanent view.
What earns the marks
- Defines quantum efficiency as the fraction of incident photons that produce a detectable signal.
- States that a CCD has much higher quantum efficiency than the eye across a wide wavelength range.
- Explains that a CCD can integrate over long exposure times, while the eye cannot accumulate light.
- Compares resolution in terms of pixel size and receptor spacing, noting that CCDs can resolve finer detail.
- Discusses convenience: CCD output is digital, permanent, storable and processable, whereas the eye gives a transient real-time image.
Common mistakes and how to put them right
Claiming the eye has no quantum efficiency.
Put it right by stating that the eye does detect photons but with low efficiency.
Saying a CCD has infinite resolution.
Put it right by linking resolution to pixel size and the diffraction limit of the telescope.
Ignoring the role of integration time.
Put it right by explaining that long exposures let a CCD build up signal from faint objects.
Exam tips
- Structure comparison answers around the three named criteria: quantum efficiency, resolution and convenience.
- Use comparative language such as higher, lower, greater or more convenient rather than describing each detector separately.
- Support each point with a brief reason, for example linking high quantum efficiency to more electrons per photon.
Try the lesson
The start of the Advantages of large diameter telescopes (A-level only): Comparison of the eye and CCD as detectors in terms of quantum efficiency, resolution, and convenience of use. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of these detectors has the highest quantum efficiency when used at the focus of a telescope?
Listen
A detector records the light gathered by a telescope. The eye detects visible light through rod and cone cells, but only a small fraction of incident photons trigger a response, so its quantum efficiency is low, often a few per cent. A CCD is a silicon array that converts photons into electrons; quantum efficiency can exceed 80 per cent across a broad range of wavelengths. The eye has limited resolution set by the spacing of receptors and cannot accumulate light, whereas a CCD has small pixels that give high spatial resolution and can integrate for long periods. CCDs are also more convenient: images are recorded digitally, stored, processed and shared, while the eye gives a real-time but non-permanent view. A strong answer earns marks for these points. Defines quantum efficiency as the fraction of incident photons that produce a detectable signal. States that a CCD has much higher quantum efficiency than the eye across a wide wavelength range. Explains that a CCD can integrate over long exposure times, while the eye cannot accumulate light. Compares resolution in terms of pixel size and receptor spacing, noting that CCDs can resolve finer detail. A common slip: Claiming the eye has no quantum efficiency. Instead, stating that the eye does detect photons but with low efficiency. Exam tip: Structure comparison answers around the three named criteria: quantum efficiency, resolution and convenience.
See it
Key words
Say what each word means before you turn the card.
Quantum efficiency
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Resolution
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Data handling and convenience
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See every definition
- Quantum efficiency
- The fraction of incident photons that produce a detectable signal, which is low for the eye but can exceed 80 per cent for a CCD.
- Resolution
- The ability to resolve fine detail, limited in the eye by the spacing of receptors and in a CCD set by small pixels.
- Data handling and convenience
- Images are recorded digitally, stored, processed and shared, while the eye gives a real-time but non-permanent view.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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