The eyes are receptors – as well as the windows to the soul. In I/GCSE Biology, the human eye can detect light intensity, from images and distinguish colours.
Structure of the eye 👀
Sclera: White part of the eye.
Cornea: A transparent structure that lets light into the eye.
Iris: Coloured ring of tissue.
Pupil: Allows light through and is black to prevent light escaping.
Choroid: Located underneath the sclera, it is a dark layer to stop light being reflected inside the eye.
Retina: Where light is transduced into electrical nerve impulses.
Rod Cells: Work well in dim light, cannot distinguish colours.
Cone Cells: Work well in bright light. Three types that respond to red, blue and greed.
Optic Nerve: Where impulses are passed from the eye to the brain.
Fovea: Centre of the retina where cones are particularly concentrated at.
To form an image on the retina, light needs to be refracted. Refraction takes place when light passes from one medium to another of different density. In the eye, this happens first at the cornea, and again at the lens, where fine focusing is done.
As a result of refraction, the image on the retina is inverted in I/GCSE Biology. The brain interprets the image right way up. The role of the iris is to control the amount of light entering the eye by changing the size of the pupil. It contains two types of muscles. Circular muscles from a ring shape in the iris and radial muscles lie like the spokes of a wheel. In bright light, the pupil is made smaller, or constricted.
Accommodation
In I/GCSE Biology, accommodation is the changes that take place in the eye which allow us to see objects at different distances. The lens is held in place by a series of fibres called suspensory ligaments, which are attached to a ring of muscle called the ciliary muscle.
When we are seeing distant obejct,
- ciliary muscles relax
- suspensory ligaments stretch
- shape of lens thin – less convex
When we are seeing near object,
- ciliary muscles contract
- suspensory ligaments slacken
- shape of lens fat – more convex
How eyes change with light? 👁️
Bright light condition: |
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Dim light condition: |
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That's the end of the topic!
Drafted by Joey (Biology)