The Eye

Will Smith

Components of the eye Cornea – a transparent protective layer which focuses light onto the

lens Lens – refracts light, focusing it onto the retina Iris – the coloured part of the eye which controls the amount of light

that enters Pupil – the hole in the iris through which light enters Retina – the surface inside the eye onto which all images are focused

on to Ciliary muscles – contract to change the shape of the lens in order to

adjust focus for different distances Suspensary ligaments – hold the lens in place and contribute to

changing lens shape

The Functioning of the eyeThe lens of the eye is convex .This means that the light rays converge after passing

through the lens and, in a healthy eye, cross at (and focus onto) the retina.

Light receptors located on the retina process the light into electrical signals and transmit these signals to the brain along the optic nerve.

The image produced is actually upside-down but this is corrected by the brain.

A healthy eye is 22mm long.

The functions of tearsPrevent the eyes from becoming too drySupplying oxygen and other nutrients to

the eyes (as there are no blood vessels)Tears contain the enzyme lysozyme which

kills bacteria, preventing infectionHealing damage to the surface of the eyeMaintaining a smooth surface of the

cornea to ensure that light is correctly refracted into the eye

The two types of lensConvex:

This type of lens converges light (i.e. closes light in)

Concave:

This type of lens diverges light (i.e. Spreads light out)

These two types of lens are important as they each treat different forms of sight loss.

Myopia – i.e. Short sightedness

Myopia is caused by an eyeball being longer than 22mm.

This means that the light rays cross before they reach the retina, therefore focusing in front of the retina.

This results in a distant objects being blurred but close objects being clear.

Treating Myopia

A concave lens is used – diverges the light rays slightly so that they focus onto the retina as normal:

Hyperopia – Long sightednessHyperopia is caused by a person’s

eyeballs being shorter than 22mm.This means that the refracted light rays

focus behind the retina, meaning that nearby objects are blurred/out of focus while distant objects can be seen clearly.

Treating hyperopiaA convex lens converges the light rays

slightly so that instead of focusing behind the retina, they focus on the retina, so a much clearer image is formed.

Summary of treatment of refractive problems

Contact lensesLike glasses, these are used to treat long

and short sightedness (as described in previous slides).

Contact lenses are prescription lenses which float on the tear film on the surface of the cornea

Laser eye surgeryLaser eye surgery is used to treat

astigmatism – when the cornea is not perfectly round, so light focuses on multiple points on the retina, leading to blurred images.

During laser eye surgery, a thin layer of cornea cells are removed in order to reshape the lens to make it perfectly round:

Before laser eye surgery:

After laser eye surgery:

The Eye

Will Smith