HOW DO WE SEE?
Seeing can be likened to recording images on a video camera. Our retina is equivalent to the film where the pictures are received, and our brain, like the electronics in the camera that process the information.
When light is received by the retina, it is converted into electrical impulses that are sent to our brain. These patterns of impulses are then compared to things we have “seen” before, and once they are recognised we actually “see”, in our brain. What is so incredible is that the retina can continue to receive and transmit new images constantly, that our brain can store and retrieve so much information, and that this can all be done so rapidly that we enjoy seamless vision.
HOW DOES THE EYE FOCUS LIGHT?
Just like a camera, our focusing system has two parts, one producing a fixed power, the Cornea, and one that is adjustable, the Chrystaline lens.
The Cornea is the clear bubble at the front of your eye. This soft clear membrane is kept inflated and in shape by the pressure of fluid inside the front section of your eye and its fixed curvature contributes about two thirds of our focusing power.
The remaining third, and our ability to keep things in focus as the get closer to us, is controlled by the Chrystaline lens, which lives just behind the Iris. This little lens is connected to a muscle which controls its shape, holding it as flat as possible for when we wish to focus in the distance, and squeezing closed to allow the lens to become fatter when we want to focus on things closer.
LIGHT AND COLOURS
At the risk of stating the obvious, we need light to enable us to see and to recognise the shapes and colours of everything around us.
We tend to think of light mostly in terms of how bright it is, and don’t actually think of it as having a specific colour. However, white light is actually made up of many different wavelengths and these different wavelengths produce different colours. For example, when we look at a rainbow we are seeing white light that has been split up into its various wavelengths so that we can see the colours separately.
The colour of an object however is dependant on the pigmentation of its surface.
As all surfaces have the ability to absorb some wavelengths of light and bounce others off, we will only see the ones that are not absorbed.
When an object’s surface absorbs all wavelengths of light it will appear black, regardless of how much light we shine on it. In the same way, a surface that reflects all wavelengths of light will appear white.
So the colours of everything we see are the result of the wavelengths that are not absorbed. For example, the skin of an apple may absorb all the wavelengths of light except for green, which bounces off, so we perceive its colour as green. However as the apple ripens and the skin pigment changes, parts of it may absorb the green wavelengths and reflect the red end of the spectrum for us to see.