3.6.4Stoichiometry
Structure of the Eye — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 4 parts Lesson 15 of 28 Unit quiz
Section 1 of 4Refracting Parts
Light is refracted at two places in the eye before it reaches the retina.
Cornea
- A transparent layer covering the front of the eye that protects it and refracts most of the light entering the eye.
Lens
- A transparent structure behind the iris that refracts light further and changes shape to fine-tune the focus.
Ciliary muscles
- Muscles that change the tension in the suspensory ligaments, which alters the shape and thickness of the lens.
Suspensory ligaments
- Fibres joining the ciliary muscles to the lens, which pull and stretch the lens as the muscles change.
Section 2 of 4Path of Light Through the Eye
1
Light enters the eye through the cornea, which refracts the rays inwards towards the centre of the eye.
2
The light passes through the pupil, an opening whose size is set by the surrounding iris.
3
The lens refracts the rays again so that they converge onto the retina at the back of the eye.
4
The image formed on the retina is real, inverted and diminished, and the brain interprets it so that it appears the correct way up.
| Part | Main function |
|---|---|
| Cornea | Protects the eye and refracts light |
| Lens | Fine focusing by changing shape |
| Iris | Controls the size of the pupil |
| Retina | Detects the image using light-sensitive cells |
Section 3 of 4Iris, Pupil and Retina
Iris, Pupil and Retina
Iris
- A ring of muscles around the pupil that controls how much light enters the eye by changing the size of the pupil.
Pupil
- The opening through which light enters; it widens in dim light and narrows in bright light.
Retina
- A layer of light-sensitive cells lining the back of the eye, which detects the image and sends signals to the brain.
Image orientation
- The image on the retina is inverted, because rays from the top of the object strike the bottom of the retina.
Section 4 of 4Exam Checklist & Tips
3.6.4 – Key Points for the Exam
- Cornea The transparent outer layer of the eye that refracts light and protects the eye.
- Lens A transparent structure that refracts light and changes shape to focus the image.
- Iris A ring of muscle that controls the size of the pupil.
- Pupil The hole in the centre of the iris through which light enters the eye.
- Retina The light-sensitive layer at the back of the eye that detects the image.
- Ciliary muscles Muscles that control the shape of the lens.
Common Mistakes
- Saying the lens does most of the refracting; most of the refraction happens at the cornea.
- Stating that the image on the retina is upright; it is inverted and the brain corrects it.
- Saying the pupil itself contracts; the iris changes the size of the pupil.
- Confusing the ciliary muscles with the iris, which controls the amount of light entering.
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