EYE

Question 1

CORNEA

Answer 1

a transparent, curved layer at the front of the eye that

refracts the light entering and helps to focus it

Question 2

Iris

Answer 2

a coloured ring of circular and radial muscle that controls

the size of the pupil

Question 3

lens

Answer 3

a transparent, convex, flexible, jelly-like structure that

refracts light to focus it onto the retina

Question 4

retina

Answer 4

a light-sensitive layer made up of rods, which detect light

of low intensity, and cones, which detect different colours

Question 5

optic nevre

Answer 5

transmits electrical impulses from the retina to the brain

Question 6

retina and blindspot

Answer 6

The internal lining at the back of the eye is the
retina and it consists of many thousands of cells that
respond to light. When light falls on these cells, they
send off nervous impulses, which travel in nerve fibres,
through the optic nerve, to the brain and so give rise
to the sensation of sight. The part of the retina lying
directly in front of the optic nerve contains no lightsensitive cells. This region is called the blind spot.

Question 7

lens pupil

Answer 7

The lens is a transparent structure, held in place
by a ring of fibres called the suspensory ligament.
Unlike the lens of a camera or a telescope, the eye
lens is flexible and can change its shape. In front of
the lens is a disc of tissue called the iris. It is the iris
we refer to when we describe the colour of the eye
as brown or blue. The iris controls how much light
enters the pupil, which is a hole in the centre of the
iris. The pupil lets in light to the rest of the eye.

Question 8

optic nerve,blind spot

Answer 8

When light falls on these cells, they
send off nervous impulses, which travel in nerve fibres,
through the optic nerve, to the brain and so give rise
to the sensation of sight. The part of the retina lying
directly in front of the optic nerve contains no lightsensitive cells. This region is called the blind spot.

Question 9

choroid ciliary body

Answer 9

The pupil looks black because all the light entering
the eye is absorbed by the black pigment in the
choroid. The choroid layer, which contains many
blood vessels, lies between the retina and the sclera.
In the front of the eyeball, it forms the iris and
the ciliary body. The ciliary body produces
aqueous humour.

Question 10

VISiON

Answer 10

Light from an object produces a focused image
on the retina (like a ‘picture’ on a cinema screen)
(Figures 14.13 and 14.17). The curved surfaces
of the cornea and lens both refract (‘bend’) the
light rays that enter the eye, in such a way that
each ‘point of light’ from the object forms a ‘point
of light’ on the retina. These points of light will
form an image, upside-down and smaller than
the object

Question 11

sclera

Answer 11

The sclera is the tough, white outer

coating.

Question 12

pupil reflex

Answer 12

The change in size of the pupil is caused by
exposure of the eye to different light intensities.
It is an automatic reaction: you cannot control it.
When bright light falls on the eye, the iris responds
by making the diameter of the pupil smaller. This
restricts the amount of light reaching the retina,
which contains the light-sensitive cells. If dim light
falls on the eye, the iris responds by making the
diameter of the pupil larger, so that as much light
as is available can reach the retina to stimulate
the light-sensitive cells.

Question 13

Circular

Answer 13

The amount of light entering the eye is controlled
by altering the size of the pupil (Figure 14.14). If the
light intensity is high, it causes a contraction in a ring of
muscle fi bres (circular muscle) in the iris. This reduces
the size of the pupil and cuts down the intensity of
light entering the eye. High-intensity light can damage
the retina, so this reaction has a protective function.

Question 14

low light

Answer 14

In low light intensities, the circular muscle of
the iris relaxes and radial muscle fi bres (which are
arranged like the spokes of a bicycle wheel) contract.
This makes the pupil enlarge and allows more
light to enter. The circular and radial muscles act
antagonistically. This means that they oppose each
other in their actions – when the circular muscles
contract they constrict the pupil and when the radial
muscles contract the pupil dilates.
The change in size of the pupil is caused by
an automatic refl ex action; you cannot control
it consciously

Question 15

acommodation

Answer 15

The eye can produce a focused image of either a near
object or a distant object. To do this the lens changes
its shape, becoming thinner for distant objects and
fatter for near objects. This change in shape is caused
by contracting or relaxing the ciliary muscle, which
forms a circular band of muscle in the ciliary body
(Figure 14.15). When the ciliary muscle is relaxed,
the outward pressure of the humours on the sclera
pulls on the suspensory ligament and stretches the
lens to its thin shape. The eye is now accommodated
(i.e. focused) for distant objects (Figures 14.15(a) and
14.16(a)). To focus a near object, the ciliary muscle
contracts to a smaller circle and this takes the tension
out of the suspensory ligament (Figures 14.15(b) and
14.16(b)). The lens is elastic and fl exible and so is
able to change to its fatter shape. This shape is better
at bending the light rays from a close object.

Question 16

retina

Answer 16

The millions of light-sensitive cells in the retina are
of two kinds, the rods and the cones (according to
shape). The cones enable us to distinguish colours, but
the rods are more sensitive to low intensities of light
and therefore play an important part in night vision
when the light intensity is not suffi cient to stimulate
the cone cells. Images formed at night appear as
shades of grey, with no bright colours detected. There
are thought to be three types of cone cell. One type
responds best to red light, one to green and one to
blue. If all three types are equally stimulated we get
the sensation of white. The cone cells are concentrated
in a central part of the retina, called the fovea
(Figure 14.11); when you study an object closely you
are making its image fall on the fovea.

Question 17

fovea

Answer 17

It is in the fovea that the image on the retina is
analysed in detail. Only objects within a 2° cone
from the eye form an image on the fovea. This
means that only about two letters in any word on
this page can be seen in detail. It is the constant
scanning movements of the eye that enable you to
build up an accurate ‘picture’ of a scene. The centre
of the fovea contains only cones: it is here that
colour discrimination occurs.

Question 18

blind spot

Answer 18

At the point where the optic nerve leaves the retina,
there are no sensory cells and so no information
reaches the brain about that part of the image which
falls on this blind spot