AQA GCSE The Eye (Biology)

The Eye

The eye is a sense organ containing receptors sensitive to light intensity and colour.

An organ is a group of tissues working together to carry out a specific function.

Light enters the eye through the cornea and is focused by the cornea and the lens onto the retina at the back of the eye. Light-sensitive cells in the retina convert the light into electrical impulses, which are transmitted along the optic nerve to the brain, where an image is formed.

Our eyes carry out two main processes to allow us to see:

1. Adaptation (adjusting to dim or bright light conditions)

2. Accommodation so that our eyes can focus on a distant or near object.

Part of eye	Structure	Function
Retina	Thin layer containing light sensitive cells (rods and cones)	Detects light and converts it into electrical impulses
Optic nerve	Bundle of nerve fibres	Transmits electrical impulses from retina to the brain
Sclera	Tough, white outer layer of the eye.	Protects the eye and maintains its shape.
Cornea	Transparent, curved front surface of the eye.	Refracts (bends) light to help focus it onto the retina.
Iris	Ring of coloured muscle around the pupil.	Controls pupil size to regulate the amount of light entering the eye.
Ciliary muscles	Ring of muscle attached to the lens via suspensory ligaments.	Contract and relax to change the shape of the lens for focusing.
Suspensory ligaments	Strong fibres connecting the lens to the ciliary muscles.	Transmit tension from the ciliary muscles to change the shape of the lens.

Adaptation for dim and bright light

The retina has rods (light-sensitive cells) and cones.

Rods are important for dim light / night vision.

Cones work in bright light and detect colour.

There needs to be the correct amount of light for both rod and cone cells to work.

Adaptation to light involves the iris adjusting the size of the pupil by contracting or relaxing its muscles, controlling the amount of light entering the eye. This is a reflex action.

Light condition	Pupil size	Reason
Dim light	Dilates (becomes larger)	Allows more light into the eye, so rods can detect the light
Bright light	Constricts (becomes smaller)	Reduces light entering the eye to prevent damage to retina

Accommodation

Accommodation is the process of changing the shape of the lens to focus on near or distant objects.

How the eye focuses on a distant object.

The light rays from a distant object are virtually parallel. This means that only a small amount of refraction is needed for the light rays to meet at the retina.

Stages:

1. The ciliary muscles relax

2.The suspensory ligaments are pulled tight

3.The lens is then pulled thin and only slightly refracts light rays.

4. Light rays meet at the retina.

See the image below, look at the width of the blue lens!

How the eye focuses on a nearby object.

Light rays from a near object need significantly more refraction to allow them to meet at the retina.

Stages:

1. The ciliary muscles contract

2.The suspensory ligaments loosen

3.The lens is then thicker and refracts light rays strongly

4. Light rays meet at the retina.

See the image below, look at the width of the blue lens!

Problems with the eyes.

There are two common defects (problems) with our eyes that can occur

1.Myopia (Short sighted)

2.Hyperopia (Long sighted)

In both cases when untreated the rays of light are not focussed onto the retina, so the image we see is blurry. These defects are treated with spectacle lenses which refract the light rays so that they do focus on the retina.

Myopia

Myopia (short-sightedness) means that a person can see nearby objects clearly, but distant objects appear blurry.

It occurs when the image is formed in front of the retina, due to the eyeball being too long or the lens being too powerful.

Myopia is corrected using a concave lens, which diverges light rays before they enter the eye so that the image is focused on the retina.

Hyperopia

Hyperopia (long-sightedness) means that a person can see distant objects clearly, but nearby objects appear blurry.

It is caused when the image is formed behind the retina, due to the eyeball being too short or the lens not being powerful enough.

Hyperopia is corrected using a convex lens, which converges light rays before they enter the eye so that the image is focused on the retina.

New technologies

Method	What it is	How it works
Spectacle lenses	Lenses worn in glasses	Concave lenses correct myopia Convex leses correct hyperopia
Contact lenses	Lenses worn directly on the eye	Can be hard or soft Do the same job as glasses by refracting (bending) light so it focuses on the retina
Laser eye surgery	Laser used on the cornea	Changes the shape of the cornea so it refracts (bends) light more or less and focuses images on the retina
Replacement lens	Artificial lens placed inside the eye	Used when the eye's natural lens is faulty e.g. cataracts