IGCSE Biology - The Eye

Conjunctiva

Mucous membrane that covers the front of the eye and lines the inside of the eyelids

Cornea

Transparent layer forming the front of the eye

Pupil

Dark circular opening in the centre of the eye which lets light through

Iris

Coloured ring of tissue which controls the amount of light entering the eye by changing the size of the pupil

Lens

Biconvex transparent body situated behind the iris that focuses light on the retina

Choroid

Dark pigmented vascular layer which stops light being reflected around the inside of the eye

Sclera

Tough outer coat of the eye

Retina

Light-sensitive layer at the back of the eye where light energy is transduced into the electrical energy of nerve impulses

Rods

Cells present in the retina that react to light, producing impulses in sensory neurones. They work well in dim light but cannot distinguish between different colours.

Cones

Cells present in the retina that react to light, producing impulses in sensory neurones. They work only in bright light and can respond to different colours of light - red, green and blue. They produce a sharp image.

Fovea

A small depression in the centre of the retina where a high concentration of cone cells can be found

Optic Nerve

Transmits impulses from sensory neurones in the retina to the brain

Ciliary muscles

Circular muscles that relax or contract to enable the lens to change shape for focussing

Suspensory ligaments

A series of fibres that hold the lens of the eye in place

Blind spot

The one area of the retina where an image cannot be formed as there are no rods or cones; this is where the optic nerve leaves the eye

Refraction

When light is bent. For example in the human eye at the air/cornea boundary and again at the lens

Circular muscles

Muscles found in the iris that contract in bright light and relax in dim light

Radial muscles

Muscles found in the iris that relax in bright light and contract in dim light

Pupil dilation

When the pupil of the eye becomes wider e.g. in response to dim light

Pupil constriction

When the pupil of the eye becomes smaller e.g. in response to bright light

Accommodation

The changes that take place in the eye which allow us to see objects at different distances