Our eyes have a number of different parts, each of which do different
things. The most important parts are:
(a) lens - this makes fine adjustments to the
focus of the view that we see
(b) cornea – the transparent part at the front of our
eye. This helps to focus the view on our retina. Most of the refraction of the light occurs
here.
(c) retina – the part of the eye that is sensitive to light. It is on the retina that
the image of the view that we are looking at is focused.
(d) optic nerve - this takes
the signals from out retina to our brain so that we can see
(e) muscles – there are
two different sets of muscles in your eyes.
(i) One set goes round the lens. These are
used in fine focusing of the eye. When these muscles contract they squeeze the lens and
so it gets fatter, this enables you to focus on things close to you. When the muscles relax
the lens becomes thinner and in this condition you can see distant objects.
(ii)The
ones at the top and bottom and on the two sides of the eye swivel your eye in its
socket so that you can look in different directions without turning your head.
(f)
iris – this is the coloured part of your eye. If someone has blue eye it is the iris that is blue.
This part of the eye controls how much light gets into your eye, it behaves just like the
diaphragm in a camera. In bright light the iris expands so that the hole in the middle of it,
the pupil, is small and only lets a small amount of light into your eye. In dim light the iris
contracts so that the pupil gets bigger allowing more light to get into your
eye.
Eyeballs
Your eyeballs are about the size of a table-tennis ball
and if you look into someone else's eyes you will see that they fit into the head and are
protected by the eyebrow above and the cheekbone below.
When your eyes
close they are covered by eyelids, and every time you blink the eyelids carry fluid
across the front of the eyes and cleans them. Rather like the windscreen washers
and wipers in a car
Cameras and eyes – similarities and
differences
The size of the pupil changes to let in different amounts of light
as does the diaphragm in a camera. However, the eye adjusts automatically unlike
some types of camera.
As we have said the muscles round the lens focus the
image, not by moving the lens but by squeezing it. Most of the refraction occurs at the
cornea, the lens is just for fine adjustment. If you try and look at something very close to
your eye you can actually feel the muscles working - you have to strain your
eyes.
The retina in the eye does the same type of job as the film in a
camera. It is a light sensitive membrane of millions of nerve endings. The main
difference is that the image is not permanent as it is on a film. The eye behaves
more like a video camera, the retina being like the CCD. However, if you look at a
very bright light and then look away you may still see an 'after image 'of the light.
The retina is composed of two types of cells, cones that are sensitive to colour and
rods that are more sensitive to detail.
Optical illusions
The
diagrams below show how we can confuse our eyes. Things are not always what they
seem. You can find some more optical illusions in the diagrams section.
Fine detail
The amount of detail that your eye can see depends on a number of
things. The brightness of the object that you are looking at, the colour of the
surroundings (if you are colour blind), and of course how far away from the object
you are.
Actually in quite dim light the pupil of your eye is large and this will
help you see detail more clearly.
Judging distance
Having two
eyes allows us to judge distances. Each eye sees the view from a slightly different
direction and the brain recognizes this. Objects near are shifted compared with ones
further away – a phenomenon known as parallax.
Our eyes are amazingly delicate
instruments but sometimes they need minor adjustments to help them work
properly. The most common defects are those of long and short sight.
A
person who has LONG SIGHT can only see things clearly that are far away from
them while a person with SHORT SIGHT can only see things clearly that are
close to them.
The first two diagrams below show how the eye focuses the image
of a near and far object on the retina by squeezing or relaxing the eye lens.
When I was talking to my friend
Carol she pointed out that people with short sight would not be able to see the
stars! The Moon would be visible but fuzzy, but because the stars look so small
and relatively faint when seen from the Earth they would become faint and not
visible if their light was blurred.
The Blind Spot
Where the
optic nerve meets the retina is a spot where there are no retina nerve ends. Any light
falling on that spot will not be detected and so it is called the Blind Spot. If so why don't we
see a blank area in front of both eyes?
It is actually to do with having two eyes. The
blind spot for one eye will not be in the same position as that for the other eye. Even if you
close one eye the blind spot is very difficult to detect. The following simple test will find it
for you.
Draw a cross and a spot on a piece of paper about 9 cm apart. Hold up
the piece of paper so that the spot is in front of your RIGHT eye. Close your LEFT eye and
look at the cross with your RIGHT eye. Move the paper towards and away from your eyes.
What happens to the DOT?
About 14% of males in the UK are colour blind. This means that they cannot see colour as clearly as other people.
I am colour blind and have difficulty in seeeing red berries amongst green leaves and reading some signs if they are written in one colour on another coloured background.
I have known someone who was completely colour blind - they saw everything in black and white!
You can test for colour blindness using the Ishihara test cards like the one shown in the picture.
I see the number 21 on this which is different from what people with normal colour vision would see.
There appears to be no connection between colour vision and the ability to see detail clearly.
Colour blindness is caused by the cone cells in your retina not working properly.