The Eye

Question 1

Describe how the rods and cones are distributed over the surface of the retina

Answer 1

Only cones at fovea,

as you move away from fovea fewer cones more rods.

Question 2

Astigmatism is caused by

Answer 2

astigmatism usually caused by an irregularity in the curvature of the cornea

Question 3

The image seen by a person with astigmatism is

Answer 3

a person with astigmatism would see an image which was less well-focussed in one particular plane [direction]

Question 4

Astigmatism is corrected using

Answer 4

defect is corrected using a (correctly orientated) cylindrical lens

Question 5

Give two quantities which must be known in order to manufacture the correcting lens for astigmatism

Answer 5

Power of lens and the angle of correction

Question 6

In terms of receptors, give the condition for two different images to be resolved by the eye.

Answer 6

Two stimulated receptors must be separated by

(at least) one unstimulated receptor

Question 7

In terms of receptors

explain why finer detail can be seen in bright light than in dim light.

Answer 7

(in bright light) cones activated .
Cones smaller than rods,
angular separation thus smaller

Question 8

State what is meant by persistence of vision

Answer 8

lights flashing at ≥ 20 Hz appear steady

Question 9

Give an example of a practical situation where persistence of vision is used to advantage

Answer 9

Cinema films or television

Question 10

Describe the distribution of receptors over the retina.

Answer 10

only cones at fovea

moving away from fovea, more rods, less cones .

Question 11

State the purpose of the iris

Answer 11

To control the intensity of light reaching retina

Question 12

State what is meant by accommodation

Answer 12

Ability of the eye/lens to (change and) focus

on different object distances

Question 13

Describe how accommodation is achieved

Answer 13

Changing the shape of the lens

Question 14

State what is meant by the principal focus

Answer 14

Principal focus is the point on the principal axis through which rays which were parallel to the principal axis pass after refraction by the lens

Question 15

State the defect of vision that would be corrected using a diverging lens

Answer 15

Myopia or short sight

Question 16

Two point sources of light are viewed by a normal eye and their images are formed at the fovea.
State, in terms of the active receptors, the conditions necessary for two separate images to be seen

Answer 16

Cones active / simulated.

Cones stimulated by images must be separated by at least 1 unstimulated cone.

Question 17

Explain what is meant by persistence of vision and state a practical situation where it is important

Answer 17

image remains after stimulus is removed

Cinema pictures, television.

Question 18

State the changes which occur in a normal eye when
the eye changes from focussing on a distant object to focussing on a near object, both objects being viewed in bright light

Answer 18

Ciliary muscles contract / suspensory ligaments relax Producing a lens
of greater power

Question 19

State the changes which occur in a normal eye when

the eye changes from viewing an object in very dim light to viewing the same object in bright light

Answer 19

Iris muscles contract and pupil becomes smaller. Cones turn on and rods become inactive

Question 20

State two differences in the perceived image of a coloured object viewed in bright white light compared to the perceived image of the same object viewed in very dim white light.

Answer 20

Colours seen in bright light, but black and white in very dim light Good detail
in bright light, but much less detail in very dim light

Question 21

A person views a flashing white light source in a darkened room. Each flash lasts for 0.01 s and initially the light is flashing at a frequency of 1 Hz.
State how the person’s perception of what is seen changes as the frequency at which the light flashes is slowly increased from 1 Hz to 40 Hz. Name the physiological process involved

Answer 21

At 1Hz, individual flashes of light seen . At some frequencies the flashes appear to join to form continuous light so that no flashing seen at 40Hz, this is called the persistence of vision.

Question 22

Describe the changes which occur in a normal eye when the eye changes from viewing a near object to viewing a distant object, both objects being viewed in bright light.

Answer 22

Ciliary muscles relax,

Producing lens of less power

Question 23

Describe the changes which occur in a normal eye as the eye changes from viewing an object in bright light to viewing the same object in very dim light

Answer 23

Cones stop working and rods start causing an

enlarged pupil

Question 24

Two bright point sources of light are just resolved as individual images when viewed by a naked eye
State the condition needed for the images to be resolved by the retina

Answer 24

The two images are formed on receptors / cells separated by at least one unstimulated receptor

Question 25

Two bright point sources of light are just resolved as individual images when viewed by a naked eye
The images are formed on the retina at the fovea.
Explain which type of receptor in the retina is being used to detect the images.

Answer 25

Cones are used as ONLY cones at fovea

Question 26

Fovea

Answer 26

Point in yellow spot of retina with highest concentration of cones

Question 27

The lens equation

Answer 27

1/f = 1/u + 1/v

Question 28

The far point

Answer 28

The furthest distance that the eye can focus comfortably, for normally sighted people it’s infinity

Question 29

The near point

Answer 29

The closest distance that the eye can focus on. For young people, this is 9cm.

Question 30

How do rods and cones work?

Answer 30

Rods and cones contain chemical pigments that bleach when light falls on them. This bleaching stimulates the cell to send signals to the brain via the optic nerve. They are unbleached (reset) by enzymes using vitamin A

Question 31

How many types of rod

Answer 31

Three: sensitive to red, green and blue light

Question 32

How many types of cone

Answer 32

one

Question 33

Why does blue light look dimmer

Answer 33

Eye is less responsive to blue light

Question 34

What is the faking of yellow light and why can it be done

Answer 34

Red and green light is combined of equal intensity, as the the electrical signal from the retina will be the same and the brain interprets it as yellow. Yellow light produces equal responses from red and green cones

Question 35

When is spatial resolution best?

Answer 35

At the yellow spot, where cones are densely packed and each cone has its own nerve fibre. There are no rods in the yellow spot.

Question 36

What is myopia?

Answer 36

Short-sighted

Question 37

When does myopia occur?

Answer 37

When the lens too powerful or the eyeball is too long, the image is formed in front of the retina

Question 38

What corrects myopia?

Answer 38

A lens of negative power, so a diverging lens

Question 39

What is hypermetropia?

Answer 39

Long sighted

Question 40

Define the power of a lens

Answer 40

reciprocal of the focal length

Question 41

What corrects hypermetropia?

Answer 41

A lens of positive power, so a converging lens