Eyes

Question 1

Explain why most of the refraction occurs

Answer 1

The greatest refraction occurs at the air/cornea boundary

because greatest difference in refractive index occurs at the air/cornea boundary

Question 2

A defective eye has an unaided far point of 2.5 m and an unaided near point of 0.20 m. A correcting lens is used to produce an aided far point at infinity.

Name the defect of vision affecting the eye.

Answer 2

Myopia or short sight

Question 3

A defective eye has an unaided far point of 2.5 m and an unaided near point of 0.20 m. A correcting lens is used to produce an aided far point at infinity.

State one possible cause of this defect of vision.

Answer 3

the eyeball is too long or the cornea is too curved/powerful.

Question 4

Give the main cause of astigmatism.

Answer 4

A non-spherical cornea

Question 5

State the effect of astigmatism on an image

Answer 5

When one plane is in focus, plane at 90° is out of focus

Question 6

State the type of lens needed to correct this defect of vision

Answer 6

A cylindrical (lens)

Question 7

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

Answer 7

The power of the lens

and the angle of correction

Question 8

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

Answer 8

Two stimulated receptors must be separated by (at least) one unstimulated receptor for resolution.

Question 9

In terms of receptors, explain why finer detail can be seen in bright light than in dim light.

Answer 9

In bright light cones are activated as well as rods, whereas in dim light only rods are activated.

Cones vision is concentrated near to the fovea and separation of them is smaller than that of rods which are more prevelant at the periphery of the retina.

Angular separation is therefore smaller for colour vision than for monochrome vision.

Question 10

State what is meant by persistence of vision.

Answer 10

Lights flashing at more than 60 Hz appear steady [or a moving image appears steady even though the stimulus is made up of flashing still shots as long as the flashing is at a rate of more than 60 per second]

Question 11

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

Answer 11

When viewing animated or cine films or television the flashing images appear as smooth image transition to us.

Question 12

Describe the distribution of receptors over the retina

Answer 12

There are only cones at fovea

As you move away from the fovea, there are more rods and less cones

Question 13

State the purpose of the iris.

Answer 13

The iris controls the intensity of light reaching retina

Question 14

Describe how this purpose is achieved when the eye is exposed to bright light.

Answer 14

The iris closes its aperture to form a small pupil.

Question 15

State what is meant by accommodation

Answer 15

Accommodation is the ability of the eye to change the power of its overall refraction system. This allows fine focus of an image on the retina for a range of object distances

Question 16

Describe how accommodation is achieved

Answer 16

Accommodation is achieved by changing the curvature of the lens by using the cilary muscles. To focus on a close object the eye lens needs to be more powerful (more curved) than when it views a distant object.

Question 17

A detailed, coloured object is illuminated by white light. Compare what is seen under high intensity light with that seen under low intensity light by an observer with normal eyesight.

Answer 17

=In bright light cones are the main photodetectors, whereas in dim light only rods are activated.
=In bright light colour vision is achieved whereas in dim light only black and white vision is possible.
=In bright light detail is clearly seen whereas there is a lack of detail in dim light.
=In bright light the image is concentrated around the optic axis but in dim light more image is at the periphery.

Question 18

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

Answer 18

Persistance of vision is where we experience the image remaining after the stimulus has been removed. This happens for a very short period of time. If a set of still pictures is flashed at us fast enough we are not aware that they are single individual images. This is because the first image is still being perceievd when the second one appears in the field of vision. There therefore does not appear to be a momentary gap between them. eg cinema pictures, television, fluorescent lights, optical illusions

Question 19

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 19

Ciliary muscles contract or suspensory ligaments relax

Producing a lens of greater power orshorter focal length

Question 20

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 20

(Iris circular muscles contract and /or radial muscles relax produces) constricted pupil or the pupil becomes smaller
Cones turn on and the signal from them is so much stronger that the rods become effectively inactive

Question 21

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 21

Colours are seen in bright light, but the image is only in monochrome (black and white) in very dim light
Good detail in bright light, but much less detail in very dim light

Question 22

Rays of light from a point source, 25 cm from a normal eye, enter the eye and are focused forming an image on the retina. State where most refraction occurs and explain why

Answer 22

The greatest refraction occurs at the boundary between the air and cornea. This is because at that interface there is the greatest change in refractive index of the materials

Question 23

Explain two changes which occur when the eye responds to a wide change in intensity of illumination from very dim light to bright light.

Answer 23

In bright light the cones are able to be used, whereas in dim light there is only a high enough light intensity for the rods to be activated. There is a greater density of cones on the central area of the retina, the rods are more widely spaced and only more concentrated than cones on the peripery of vision, therefore images seen in bright light have higher definition.
In bright light the iris reacts to make the pupil smaller This limits the intensity of light falling on retina so that the cones are not ‘bleached out’ because they recieve above their maxium intensity of light.