The Eyes

• The structure of the eye responds to changes in light intensity with the iris reflex.
• Accommodation is a reflex that changes the refractive power of the lens so that you can see both near and distant objects.
• Glasses can help when the accommodation process doesn't work properly.
• The cornea, which is completely transparent, allows all the light to pass through it and has no blood vessels, as it needs all the oxygen it requires from the outside air.
• The cornea and the lens both refract or bend light.
• The cornea causes all the light that passes through it to refract or bend by a certain amount.
• The ciliary muscles and suspensory ligaments control the shape of the lens.
• The iris, which is the coloured part of the eye, controls how big or small the pupil is.
• When light from an object hits the eye, it has to be focused onto the spot of the retina called the fovea.
• The pupil, which is not a structure but a gap in the middle of the iris, allows the light to pass through to the lens.
• The cornea always attracts light by the same amount.
• The role of the lens is to fine-tune the refraction so that the light rays always converge exactly on the fovea regardless of whether the object is really far away or very close.
• The job of the lens is to refract or bend light, and it can change its shape which allows it to control how strongly it refracts the light, helping it to always focus the light perfectly onto the retina at the back of the eye.
• If an object is close to us, the light from its surface has to be refracted a lot which means that even once it's been refracted by the cornea it still needs a powerful lens to refract it some more.
• The retina is made up of two different types of receptor cells, cone cells which are sensitive to the color of light and allow us to see in color, and rod cells which are more sensitive to light but only allow us to see in black and white.
• The lens needs to be short and fat so that it's more curved which means that it can refract more strongly.
• The fovea is a special spot on the retina which is full of only cone cells and it's this region that we try to focus light on so that we can see things most clearly.
• The optic nerve takes all of the impulses generated by the receptor cells and transmits them to the brain.
• Whenever you look at a nearby object, the ciliary muscle contracts and it actually contracts inwards towards the lens because it's now closer to the lens.
• The iris reflex controls the size of the pupil making it smaller in bright light conditions and larger in low light conditions.
• The suspensory ligaments slacken which means they become loose and because they're no longer pulling tight on the lens the lens is free to return to its natural fatter shape so it can now refract light more strongly.
• The iris is made up of two different types of muscles, circular muscles that stretch around the pupil like circles and radial muscles that stretch from the inside to the outside like the radius of a circle.
• For distant objects, the light doesn't need to be refracted as strongly which means that the lens doesn't need to do as much because the cornea has already refracted the light most of the way to reduce the refractive power of the lens.
• To make the pupil smaller in bright light, the circular muscles contract squeezing the pupil smaller and the radial muscles have to be relaxed so that they can be stretched longer.
• If it's dark and the pupil needs to let in more light, the circular muscles relax and the radial muscles now contract making them shorter and pulling the pupil open.
• To achieve this, the ciliary muscle relaxes moving away from the lens and pulling these expensive ligaments taut which stretches the lens out.
• People with long-sightedness, also known as hyperopia, can see long distances fine but are unable to focus on nearby objects.
• Glasses containing convex lenses help by providing some extra refracting power so that the eye is then able to focus the light onto the retina properly.
• In short-sighted people, the lens refracts light too much which means that light from distant objects which doesn't normally need to be refracted very much ends up being over-refracted so that it focuses and forms an image before the retina.
• Glasses containing concave lenses refract light outwards and so counteract the over-refraction of the lens which allows the light to focus on the retina like we want.