The Eye and Vision Defects

The eyes

• The sclera is the tough supporting wall of the eye.
• The cornea is the transparent outer layer found at the front of the eye. It refracts light into the eye.
• The iris contains muscles that allow it to control the diameter of the pupil (the hole in the middle) and therefore how much light enters the eye.
• The lens focuses the light onto the retina (which contains receptor cells that are sensitive to light and colour).
• The shape of the lens is determined by the ciliary muscles and suspensory ligaments.
• The optic nerve carries impulses from the receptors onto the brain.
• Bright light can damage the retina, so you have a reflex to stop it. When light receptors in the eye detect very intense light, a reflex is triggered that makes the pupil smaller. The circular muscles in the iris contract and the radial muscles relax. This reduces the amount of light able to enter the eye.
• The opposite process happens in dim light: the radial muscles contract and the circular muscles contract to make the pupil wider.
• The eye focuses light on the retina by changing the shape of the lens. This is known as accomodation.
• To look at near objects, the ciliary muscles contract, which slackens the suspensory ligaments. The lens becomes more curved so the amount by which it refracts light is increased.
• To look at distant objects, the ciliary muslces relax, allowing the suspensory ligaments to tighten up, making the lens go thinner, causing less light refraction in the eye.
• If a person cannot refract the light by the right amount (so that it focuses on the retina), the person will be short…