The Eye Flashcards
Describe how the rods and cones are distributed over the surface of the retina
Only cones at fovea,
as you move away from fovea fewer cones more rods.
Astigmatism is caused by
astigmatism usually caused by an irregularity in the curvature of the cornea
The image seen by a person with astigmatism is
a person with astigmatism would see an image which was less well-focussed in one particular plane [direction]
Astigmatism is corrected using
defect is corrected using a (correctly orientated) cylindrical lens
Give two quantities which must be known in order to manufacture the correcting lens for astigmatism
Power of lens and the angle of correction
In terms of receptors, give the condition for two different images to be resolved by the eye.
Two stimulated receptors must be separated by
(at least) one unstimulated receptor
In terms of receptors
explain why finer detail can be seen in bright light than in dim light.
(in bright light) cones activated .
Cones smaller than rods,
angular separation thus smaller
State what is meant by persistence of vision
lights flashing at ≥ 20 Hz appear steady
Give an example of a practical situation where persistence of vision is used to advantage
Cinema films or television
Describe the distribution of receptors over the retina.
only cones at fovea
moving away from fovea, more rods, less cones .
State the purpose of the iris
To control the intensity of light reaching retina
State what is meant by accommodation
Ability of the eye/lens to (change and) focus
on different object distances
Describe how accommodation is achieved
Changing the shape of the lens
State what is meant by the principal focus
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
State the defect of vision that would be corrected using a diverging lens
Myopia or short sight
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
Cones active / simulated.
Cones stimulated by images must be separated by at least 1 unstimulated cone.
Explain what is meant by persistence of vision and state a practical situation where it is important
image remains after stimulus is removed
Cinema pictures, television.
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
Ciliary muscles contract / suspensory ligaments relax Producing a lens
of greater power
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
Iris muscles contract and pupil becomes smaller. Cones turn on and rods become inactive
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.
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
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
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.
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.
Ciliary muscles relax,
Producing lens of less power
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
Cones stop working and rods start causing an
enlarged pupil
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
The two images are formed on receptors / cells separated by at least one unstimulated receptor
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.
Cones are used as ONLY cones at fovea
Fovea
Point in yellow spot of retina with highest concentration of cones
The lens equation
1/f = 1/u + 1/v
The far point
The furthest distance that the eye can focus comfortably, for normally sighted people it’s infinity
The near point
The closest distance that the eye can focus on. For young people, this is 9cm.
How do rods and cones work?
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
How many types of rod
Three: sensitive to red, green and blue light
How many types of cone
one
Why does blue light look dimmer
Eye is less responsive to blue light
What is the faking of yellow light and why can it be done
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
When is spatial resolution best?
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.
What is myopia?
Short-sighted
When does myopia occur?
When the lens too powerful or the eyeball is too long, the image is formed in front of the retina
What corrects myopia?
A lens of negative power, so a diverging lens
What is hypermetropia?
Long sighted
Define the power of a lens
reciprocal of the focal length
What corrects hypermetropia?
A lens of positive power, so a converging lens
