Biology Edexcel Topic 2 (The eye)

2.15B to 2.17B

Structure of the eye

Cornea, iris, ciliary muscle, lens, suspensory ligaments, retina, optic nerve

Structure and function of the cornea

Transparent outer covering of the eye which refracts the light entering the eye

Structure and function of iris

Pigmented ring of circular muscles and radial muscles which controls the size of the pupil to alter how much light enters the eye

What is the pupil?

A hole in the iris centre that allows light rays to enter the eye

Structure and function of the lens

Transparent, bi-convex (curved outwards on both sides) structure which is attached to a ring of ciliary muscle by the suspensory ligaments and refracts light, focusing it onto the retina

Function of ciliary muscles and suspensory ligaments

Changes the shape of the lens to focus light onto the retina

Structure and function of the retina

Light sensitive layer composed of rod and cone cells which converts light energy into neural signals which are sent to the brain via the optic nerve

What are rod cells?

Cells in the retina that are sensitive to low light intensity (dim light)

What are cone cells?

Cells found in the retina that are sensitive to high light intensity (bright light) and can detect different colours

Function of the optic nerve

It transmits nerve impulses to the brain from the retina

How does dim light affect the size of the pupil?

Light receptors (rods) detect the dim light, the circular muscles relax and the radial muscles contract, which makes the pupil dilate so more light enters the eye

How does bright light affect the size of the pupil?

Light receptors (rods) detect bright light, the circular muscles contract and the radial muscles relax, which makes the pupil contract so less light enters the eye

Why is the iris reflex important?

It prevents bright light from damaging the retina

What is accommodation?

Process by which the elastic lens changes its shape (with the aid of ciliary muscles and suspensory ligaments) to focus on near or distant objects - light is focused onto the retina

How does the eye focus on near objects?

The ciliary muscles contract and suspensory ligaments slacken which means the lens become more convex (more rounded) so the light is refracted more and the light rays focus on the retina

How does the eye focus on distant objects?

The ciliary muscles relax and suspensory ligaments tighten, which means the lens becomes less convex (less rounded) so the light is refracted less and the light rays focus on the retina

What is long-sightedness?

When somebody can focus on distant objects clearly but not on near objects

What are the causes of long-sightedness?

Eyeball is too short or the lens is less elastic (age related) so the light rays don't focus on the retina (they converge behind the retina)

How is long-sightedness treated?

Using a convex lens (causes light rays to converge) in glasses or contact lenses, replacing the lens in the eye, or laser eye surgery

What is short-sightedness?

When a person can focus on near objects clearly but not on distant objects

What are the causes of short sightedness?

Eyeball is too long or lens is too thick and too rounded so light rays don't focus on the retina (the converge in front of the retina)

How is short-sightedness treated?

Using a concave lens (causes light rays to diverge) in glasses or contact lenses, replacing the lens in the eye, or laser eye surgery

What are cataracts?

A cloudy patch that forms on the lens of the eye which makes the vision become blurry, makes it difficult to see the intensity of colours, and can cause problems seeing in bright light (glare)

How are cataracts treated?

The clouded lens is exchanged for a synthetic lens during surgery

What is colour blindness?

A deficiency of the eye that makes is difficult to distinguish between colours (e.g. individuals with red-green colour blindness have difficulty differentiating between red and green)

What is the cause of colour blindness?

Damage to cone cells in the retina or not enough cone cells in the retina