IGCSE Biology / Coordination
The structure of the eye revision notes
Edexcel International GCSE Biology (4BI1), chapter 6: Coordination, specification point 2.91. Free lesson.
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The eye lets light in, focuses it and detects it. Knowing how its parts work helps you label the eye and explain how vision is affected by light levels and the position of receptors.
How light enters the eye
Light passes through the front of the eye before reaching the retina. The cornea does most of the bending, or refraction, of the light. The lens is flexible and fine-tunes the focus. This means both the cornea and lens are involved when light is focused on the retina. The iris is a ring of muscle that controls how much light enters the eye. The pupil is the hole in the middle of the iris. It is a gap, not a structure.
What the parts at the back of the eye do
The retina lines the back of the eye. It contains light receptors, and the image forms there. The optic nerve carries impulses from the retina to the brain. The ciliary muscle and suspensory ligaments hold the lens and change its shape. The choroid is a dark layer just outside the retina. Its blood vessels supply the retina, and its pigment absorbs light that passes the receptors so it cannot bounce around inside the eye and blur vision. The sclera is the tough white outer coat. It protects the eye and helps it keep its shape.
How rods and cones detect light
The retina contains two kinds of light receptor: rods and cones. They detect different things and work best in different light levels. Rods detect light and dark only, so they do not detect colour. They work in dim light and are spread across the whole retina. Cones detect colour and need bright light to work. They are concentrated at the fovea, directly opposite the pupil. The fovea receives light from whatever you look at directly. This is why colour vision is best when you look straight at something.
Why the blind spot has no vision
The blind spot is the point where the optic nerve leaves the retina. There are no rods or cones there, so light landing on this spot is not detected. In dim light, a faint object can be easier to see if you look slightly to one side. Looking directly at it puts its light on the fovea, which has mainly cones. Cones need bright light, so they do not work well in dim conditions. Looking slightly aside puts the light on rods elsewhere on the retina. Rods work in dim light.