How the eye works

Everything we see is a result of the projection of light onto the retina, a thin piece of tissue at the back of the eye.  It converts the light into signals interpreted by the brain as images.  All of the other parts of the eye work together so that the light is focused correctly, ensuring that the image sent to the brain is accurate.  Vision problems occur when any of these important parts are damaged or not operating normally.  It is therefore easiest to discuss those problems when you can compare them with a normal eye and see what part of the chain of the events has broken down.  Here we will watch the way light journeys through the various components of the eye to its destination:  the retina.

When light enters the eye, the first thing it encounters is the cornea, a clear film of five layers of tissue.  Transparency is the most crucial quality of the cornea, and so it has no blood vessels—all of the necessary nutrients are diffused through fluids on either the inside or the outside of the cornea and through a transparent protective cover called the conjunctiva.  The cornea’s two main functions are to protect the eye from toxins and particles that could damage the eye and to act as the primary window for light entering the eye, reflecting and focusing the light onto the lens, light’s next stop in its journey. 

Between the cornea and the lens, however, it is important to mention the iris and the pupil.  The iris is the colored part of the eye, and the pupil is actually just a small hole in the iris, even though it appears to be black.  Light reflected from the cornea passes through the pupil and onto the lens, and the iris controls just how much of that light is allowed through.  In bright areas the pupil does not need to be very large for clear vision, while in darkness the pupil will dilate (become larger) to optimize the amount of light that can get back to the lens and the retina.  To see this happening in real time, just spend a few minutes outside on a sunny day and then walk inside and look in a mirror.  A few seconds after coming inside your pupil will be small, but given some time to assimilate it will adjust to the darker room and  expand.

The light that passes through the pupil is reflected onto the lens next.  Much like a camera lens, both the cornea and the lens have the job of correctly focusing the light so that the image projected onto the retina is as clear as possible.  The lens is transparent and fibrous and controlled by tiny muscles that flatten or accommodate it depending on whether or not the eye is focusing on something that is far or near.  The change in the shape of the lens also affects the size of the pupil, which gets larger when the lens is flat and focusing on something far away and gets smaller when light is reflecting off an object that is close and the lens is more convex.  The pupil therefore effectively has an infinite number of shapes, as it is affected by the amount of light entering the eye and the distance away from the object in focus. 

Once the light has been processed by the lens it is then bent and reflected as an upside-down image onto a single point on the last of the ten layers of the retina.  This, however, is not quite the end of the journey, because from that layer the image is then projected onto the ninth layer, the photoreceptor layer.  This is the part of the retina that is able to interpret the image that up until now has just been passed through and processed by the other parts of the eye.  There are two categories of photoreceptors, rods and cones, and it is they that compress the image into a message that is sent through the optic nerve to the brain, where finally the inverted image becomes the right-side-up one that you see.

The process of vision is very complex and requires that every participant function exactly as it was intended, because any alteration, however slight, can result in impaired vision.  It is no wonder, therefore, that vision problems are so common and so varied, or that there are people who spend their careers devoted to solving them!

Ref  http://www.tedmontgomery.com/the_eye/eyephotos/index-grphc.html