The eye is often compared to a basic camera, and indeed the very first camera was designed with the concept of the eye in mind. We can reduce the complex process that occurs to process light into vision within the eye to a relatively basic sequence of events. First, light passes through the cornea, which refracts the light so that it enters the eye in the right direction, and aqueous humour, into the main body of the eye through the pupil. The iris contracts to control pupil size and this limits the amount of light that is let through into the eye so that light-sensitive parts of the eye are not damaged. The pupil can vary in size between 2mm and 8mm, increasing to allow up to 30 times more light in than the minimum. The light is then passed through the lens, which further refracts the light, which then travels through the vitreous humour to the back of the eye and is reflected onto the retina, the centre point of which is the macula. The retina is where the rods and cones are situated, rods being responsible for vision when low levels of light are present and cones being responsible for colour vision and specific detail. All the light information that has been received by the eye is then converted into electrical impulses by a chemical in the retina called rhodopsin, also known as purple visual, and the impulses are then transmitted through the optic nerve to the brain where they are perceived as ‘vision’. The eye moves to allow a range of vision of approximately 180 degrees and to do this it has four primary muscles which control the movement of the eyeball. These allow the eye to move up and down and across, while restricting movement so that the eye does not rotate back into the socket.
Rods are the light-sensitive cells in our eyes that aid our vision in low levels of light. Rods are blind to colour and only transmit information mainly in black and white to the brain. They are far more numerous with around 120 million rods present in every human eye compared to around 7 million cones. Cones are responsible for perceiving colour and specifi c detail. Cones are primarily focused in the fovea, the central area of the macula whereas rods mainly surround the outside of the retina. Cones work much better in daylight as light is needed to perceive colour and detail. Colour is not actually inherent in any object. We only see colour because objects absorb some colour from light, and refl ect others. It is the refl ected ones that we see and that give an object a set ‘colour’. Therefore, for example, grass is not green, it purely absorbs all other colours in light and refl ects back green. If an object refl ects all colours we will see it as white, if it absorbs all colours we see it as black. We use cones to perceive colour as rods are blind to colour. Nerve fi bres Bipolar cells Synapsis Receptors Rods Cones Pigmented cells Ganglion cells Light
Colour is not actually inherent in any object. We only see colour because objects absorb some colour from light, and refl ect others. It is the refl ected ones that we see and that give an object a set ‘colour’. Therefore, for example, grass is not green, it purely absorbs all other colours in light and refl ects back green. If an object refl ects all colours we will see it as white, if it absorbs all colours we see it as black. We use cones to perceive colour as rods are blind to colour.
