The Eye - The Eye: Its Responses - (GCSE Biology)

The Eye: Its Responses

The Pupil Reflex

Bright light can damage the retina so the pupil reflex helps change the diameter of the pupil to control how much light enters the eye:

• The antagonistic action of muscles in the iris control pupil diameter. There are radial and circular muscles in the iris. When one contracts, the other relaxes.
• In bright light, the pupil reflex decreases the diameter of the pupil. This decreases the amount of light that enters the eye. The circular muscles contract and the radial muscles relax.
• In dim light, the pupil reflex increases the diameter of the pupil. This increase the amount of light that enters the eye. The circular muscles relax and the radial muscles contract.

Accommodation

The lens refracts the light entering the eye so that it can converge on the retina. This allows the object to be brought into focus.

Accommodation changes the shape of the lens to adjust how much refraction is taking place. This is because nearby objects need more refraction to be seen than far away objects. It is a reflex action.

Nearby Objects

To focus on near objects:

• The suspensory ligaments relax
• The ciliary muscles contract
• The lens becomes fatter and more curved

If the lens is fatter, it has a greater refractive effect, so the light refracts and bends even more. Nearby objects have less distance and time to come into focus, therefore an increased refractive effect of the lens is required.

Far Objects

To focus on far objects:

• The suspensory ligaments contract
• The ciliary muscles relax
• The less becomes thinner and less curved

If the lens is thinner, the refractive effect is reduced, causing the light to bend less. Far away objects have more time and a greater distance to come into focus, therefore a reduced refractive effect of the lens is required.

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Defects of the Eye

Accommodation is a very fine process, but in many individuals it can go wrong leading to defects such as myopia (short sightedness) and hyperopia (long sightedness) in which rays of light do not focus on the retina.

The majority of those who wear glasses require the glasses for the purpose of correcting refraction.

Myopia

• Myopia is short sightedness. Myopia is a struggle to see far away objects. In those with myopia, there is too much refraction. Therefore the light rays end up crossing each other before the retina, and the light focuses at a point too far in front of the retina.
• Myopia could be due to a thin lens. Myopia can occur because the lens may be too thin, which means that there is lots of refraction. Alternatively, the eyeball may be too long, which means that the light
  rays cross before they reach the end of the eyeball (and the retina).
• Corrective concave lenses can be used. Corrective concave lenses can be used to correct the refraction. They make the incoming light rays diverge (move away from each other) before they hit the lens, so
  that the rays end up focusing on the retina.
• Lenses are found in glasses or contact lenses. Corrective contact lenses are often fitted into spectacles. However, spectacles can be uncomfortable and aesthetically unpleasing. Therefore, contact lenses are now offered to myopics. Contact lenses can be hard or soft. Soft lenses are more comfortable, but are less resistant and often have to be disposable, whereas hard lenses can generally be worn for longer periods of time.

Hyperopia

• Hyperopia is long sightedness. Hyperopia is a struggle to see nearby objects. In those with hyperopia, there is too little refraction. Therefore the light rays are still wide apart when they reach the retina. They would be destined to cross at a point behind the retina.
• Hyperopia could be due to a thick lens. Hyperopia can occur because the lens may be too thick, which means that there is little refraction. Alternatively, the eyeball may be too short, which means that the light rays do not cross by the time they reach the end of the eyeball (and the retina).
• Corrective concave lenses can be used. Corrective convex lenses can be used to correct the refraction. They make the incoming light rays converge (move closer to each other) before they hit the lens, so that the rays end up focusing on the retina.
• Lenses are found in glasses or contact lenses. Like short-sightedness, lenses can be found in glasses or contact lenses. People with hyperopia often use glasses for reading (close vision) but not in general life.

Treatments for Hyperopia and Myopia

There are several treatments for hyperopia and myopia:

• Glasses – glasses have corrective concave or convex lenses, as discussed above
• Contact lenses – contact lenses are more user-friendly substitutes to glasses, as discussed above. Contact lenses can be hard or soft. Hard contact lenses are more durable and resistant, whilst soft contact lenses are more comfortable.
• Laser surgery – laser eye surgery is now an option for those with these disorders. They provide a longer term alternative to contact lenses or spectacles. The surgery is not risk free, as it involves using a laser to change the shape of the cornea, so you can get infection or even go blind (although this is rarer now). It is more commonly used for myopia, however some of those with hyperopia can benefit from the treatment.
• Replacement lenses – artificial lenses can also be implanted to replace the existing lens of the person. This is a long term alternative to spectacles or contact lenses. This involves surgery which requires a cut into the cornea. The inserted lens will be of the correct shape to increase or decrease the refraction based on whether the person is long or short sighted.

Other Defects of the Eye

Cataracts

• Cataracts can cause blurred vision. Cataracts are cloudy patches that form on the lens. They prevent light from entering the eye as they normally would causing blurred vision. Other symptoms also include difficulty seeing in bright light and colours appearing less vivid.
• A cataract can be treated by replacing the lens. The faulty, patchy lens can be replaced with a new, artificial lens.

Colour Blindness

• Colour blindness can cause difficulty in differentiating between certain colours. Most commonly, those with colour blindness can’t differentiate between red and green. This is due to the red or green cones in the retina being faulty.
• There is no cure for colour blindness as of yet. There is currently no way to replace these faulty cones.