3.6.6Stoichiometry
The Eye and the Camera — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 4 parts Lesson 17 of 28 Unit quiz
Section 1 of 4Matching Parts
Both systems use a convex lens to form an image on a detector.
Lens
- Both use a convex lens to refract light, but the eye lens changes shape while a camera lens keeps a fixed shape and moves.
Detector
- The retina detects the image in the eye, while a CCD sensor or photographic film detects it in a camera.
Brightness control
- The iris controls the size of the pupil in the eye, while the aperture controls how much light enters a camera.
Focusing
- Ciliary muscles change the lens thickness in the eye, while a camera lens moves nearer to or further from the sensor.
| Feature | Eye | Camera |
|---|---|---|
| Lens | Variable focus convex lens | Fixed focus convex lens |
| Detector | Retina | CCD sensor or photographic film |
| Brightness control | Iris and pupil | Aperture |
| Image formed | Real, inverted and diminished | Real, inverted and diminished |
Section 2 of 4Forming the Image
1
Light from the object passes through the convex lens and converges onto the detector surface.
2
In both systems the image formed is real, inverted and diminished compared with the object.
3
A CCD camera converts the light into electrical signals that can be stored, viewed on a screen or sent to a computer for printing.
4
A film camera records the image chemically on photographic film, which must be developed in a dark room before the photograph can be seen.
Section 3 of 4Comparing the Two
Comparing the Two
Focusing
- The eye focuses automatically by changing lens shape, while a camera lens must be moved for close and distant objects.
Brightness
- Both control the amount of light reaching the detector, the eye with the iris and the camera with the aperture blades.
Output
- A CCD camera stores the image as digital data that can be copied or printed, whereas film needs chemical processing.
Limitation
- The eye can only detect the visible spectrum, while a camera's detector and film each respond to a range that depends on its design.
Section 4 of 4Exam Checklist & Tips
3.6.6 – Key Points for the Exam
- Convex lens A lens that converges parallel rays to a focus and is used in both the eye and the camera.
- CCD A charge-coupled device sensor that converts light into electrical signals in a digital camera.
- Aperture The opening in a camera lens system through which light passes, whose size controls brightness.
- Retina The light-sensitive layer at the back of the eye that detects the image.
- Variable focus A lens whose focal length can be changed, as in the human eye.
Common Mistakes
- Saying the image on the retina is upright; the image is inverted and the brain corrects it.
- Stating that a camera lens changes shape in the same way as the eye lens.
- Confusing the iris, which controls brightness in the eye, with the lens, which focuses the image.
- Describing the image in a CCD camera as virtual; it is a real image that can be captured on the sensor.
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