The eye
The eye is a remarkable structure. It allows us to see our world in extremely high resolution,
in three dimensions, and in colour, at night and during the day. Many animals can do one of
these things far better than humans. For example, dogs have much better night vision.
However, few animals are as good in all areas as humans.
Figure 1. A cross-section of the eye.
Label the structure of the eye:
Complete the table below on the role of the different parts of the eye:
Structure in the eye
Sclera
Cornea
Pupil
Iris
Lens
Retina
Optic nerve
Role
The size of the iris is controlled by another reflex arc. The iris responds to changes in light
intensity: it will rapidly open or close, altering the amount of light entering the eye.
Pupil reflex arc
Describe the pupil reflex arc.
The pupil reflex arc prevents damage to the eye. This response is involuntary and happens
rapidly.
Muscles often work in pairs called antagonistic pairs. As one muscle contracts, the other
muscle relaxes. The radial and circular muscles in the iris are an example of an antagonistic
pair.
Focusing the eye
The light from a distant object enters the eye at a different angle to the light from an object
close by.
The eye must refract the light from these objects onto the retina, so that the images appear in
focus.
Using the diagram below, describe what happens to the ciliary muscles when viewing:
(i) close objects
(ii) distant objects
Figure 1 shows how the ciliary muscles contract or relax to change the shape of the lens.
This process is called accommodation.

Someone who is short sighted cannot clearly see objects that are far away, because the
light is not focused onto their retina. Glasses with concave lenses are used to correct
short-sightedness.

Someone who is long sighted cannot clearly see objects that are close to them.
Glasses with convex lenses are used to correct long-sightedness.
Upside down vision
Have you ever heard that we see upside down? This is actually true. When light enters our
eyes, it is refracted through the cornea and the lens before it strikes the retina. As it passes
through the lens, the light is inverted. Why do we not see things upside down then?
Inverted vision.
Special glasses exist that can invert the image entering the eyes, so you see upside down.
Rods and cones
The retina contains receptors that detect and process the light entering the eye and create the
nerve impulses that are sent to the brain. There are two types of receptors that have role in
vision. What are they?
Where are cones located?
What is the function of cones?
How many different types of cones are there?
For example, if we look at yellow light, our green and red cones respond; our brain interprets
these impulses to ‘see’ yellow. If we look at magenta light, our red and blue cones
respond. Refer to picture on powerpoint to see how this works.
Where are rods found?
Why do stars appear brighter if you do not look at them directly?
What is the 'blind spot'?
The optic nerve connects to the eye at the very centre of the retina. This area is called the
‘blind spot’, because there are no rods or cones; this means that light hitting this area cannot
be detected. When you look straight at something, you do not actually see the centre of the
image. However, your brain interpolates (adds in) what it thinks should be there, based on
what the retina has detected around the blind spot; your brain also uses information from your
other eye to fill in the gaps. This is why you do not have a blank space in the centre of your
vision.
Figure 2. An illustration of a human eye, showing the blood vessels, the fovea and
the optic nerve. The area highlighted in yellow shows where the blind spot is located.
Blind spot
1. Close your left eye.
2. Look closely at the cross in Figure 3. Do not look at the circle but try to remain aware
of it in your peripheral vision.
3. Move your head slowly towards the screen.
Figure 3. Follow the instructions above. At a particular distance from the screen, you will
find that you cannot see the circle.
You should have found that, at a certain distance from the screen, you could not see the black
circle. At this point, the light reflecting from the circle was hitting the blind spot on your
retina; as a result, it was not detected.
Colour blindness
Some people are born with a genetic condition that causes abnormal colour vision. These
people may not have one of the three types of cone (red, blue or green), or their cones may
not work properly.
One of the most common types of colour blindness is red-green colour blindness. In this
condition, the person does not have either red cones or green cones, or one type of cone does
not work so well.
The test for red-green colour blindness is shown in Figure 4.
Figure 4. Tests for colour blindness. (For a qualified medical diagnosis, consult an eye care professional.)