How we see
When you take good care of your eyes, you take good care of yourself.
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 hrough 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 and Cones
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 specific 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
When light enters the eye, it first passes through the cornea, then the aqueous humor, lens
and vitreous humor. Ultimately it reaches the retina, which is the light-sensing structure of
the eye. The retina contains two types of cells, called rods and cones. Rods handle vision in
low light, and cones handle color vision and detail. When light contacts these two types of
cells, a series of complex chemical reactions occurs. The chemical that is formed (activated
rhodopsin) creates electrical impulses in the optic nerve. Generally, the outer segment of
rods are long and thin, whereas the outer segment of cones are more, well, cone shaped.
Below is an example of a rod and a cone:
The outer segment of a rod or a cone contains the photosensitive chemicals. In rods, this
chemical is calledrhodopsin; in cones, these chemicals are called color pigments. The
retina contains 100 million rods and 7 million cones. The retina is lined with black pigment
called melanin -- just as the inside of a camera is black -- to lessen the amount of reflection.
The retina has a central area, called the macula, that contains a high concentration of only
cones. This area is responsible for sharp, detailed vision.
When light enters the eye, it comes in contact with the photosensitive chemical rhodopsin
(also called visual purple). Rhodopsin is a mixture of a protein called scotopsin and 11-
cis-retinal -- the latter is derived from vitamin A (which is why a lack of vitamin A causes
vision problems). Rhodopsin decomposes when it is exposed to light because light causes a
physical change in the 11-cis-retinal portion of the rhodopsin, changing it to all-trans retinal.
This first reaction takes only a few trillionths of a second. The 11-cis-retinal is an angulated
molecule, while all-trans retinal is a straight molecule. This makes the chemical unstable.
Rhodopsin breaks down into several intermediate compounds, but eventually (in less than a
second) forms metarhodopsin II (activated rhodopsin). This chemical causes electrical
impulses that are transmitted to the brain and interpreted as light. Here is a diagram of the
Activated rhodopsin causes electrical impulses in the following way:
1. The cell membrane (outer layer) of a rod cell has an electric charge. When light activates
rhodopsin, it causes a reduction in cyclic GMP, which causes this electric charge to
increase. This produces an electric current along the cell. When more light is detected, more
rhodopsin is activated and more electric current is produced.
2. This electric impulse eventually reaches a ganglion cell, and then the optic nerve.
3. The nerves reach the optic chasm, where the nerve fibers from the inside half of each retina
cross to the other side of the brain, but the nerve fibers from the outside half of the retina
stay on the same side of the brain.
4. These fibers eventually reach the back of the brain (occipital lobe). This is where vision is
interpreted and is called the primary visual cortex. Some of the visual fibers go to other
parts of the brain to help to control eye movements, response of the pupils and iris, and
Eventually, rhodopsin needs to be re-formed so that the process can recur. The all-trans
retinal is converted to 11-cis-retinal, which then recombines with scotopsin to form rhodopsin
to begin the process again when exposed to 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 reflected 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
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.
Ishihara based his test on pseudo-isochromaticism, but with the intention of
delivering results that were more easily interpreted and thus more reliable. Almost
nine decades on from its first edition, the Ishihara test remains widely used, able to
quickly screen for colour vision defects that other, more exacting tests can then
elucidate in detail. The Ishihara test can only detect the more common red-green
colour vision deficiencies (not the rarer blue ones), and then with only limited
precision. A mild form of red-green deficiency occurs when either the red or green
sensitive photopigment in the retina has an altered response to colour; this results in
reduced discrimination between the colours red and green. A more severe deficiency
occurs when either the red or green photopigment is missing entirely.
Normal colour vision uses all three types of light cones correctly and is known as
trichromacy. People with normal colour vision are known as trichromats.
The different anomalous conditions are protanomaly, which is a reduced sensitivity
to red light, deuteranomalywhich is a reduced sensitivity to green light and is the
most common form of colour blindness and tritanomaly which is a reduced
sensitivity to blue light and is extremely rare.
People with deuteranomaly and protanomaly are collectively known as red-green
colour blind and they generally have difficulty distinguishing between reds, greens,
browns and oranges. They also commonly confuse different types of blue and purple
People with reduced blue sensitivity have difficulty identifying differences between
blue and yellow, violet and red and blue and green. To these people the world
appears as generally red, pink, black, white, grey and turquoise.
People with monochromatic vision can see no colour at all and their world consists of
different shades of grey ranging from black to white, rather like only seeing the world
on an old black and white television set. Achromatopsia is extremely rare, occuring
only in approximately 1 person in 33,000 and its symptoms can make life very
difficult. Usually someone with achromatopsia will need to wear dark glasses inside
in normal light conditions.