Medical Physics: The Eye

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  • Created by: LZ95
  • Created on: 01-04-14 20:16
What controls the size of the pupil?
The muscular iris
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Which muscles alter the shape of the lens?
Ciliary muscles
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What happens when you look at an object close up?
The lens fattens (ciliary muscles contract) thereby becoming optically stronger
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What happens when you look at objects in the distance?
Lens gets thinner and ciliary muscles relax
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What is the main boundary at which refraction takes place?
The air cornea boundary
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Where are the rods and cones located?
At the back of the retina
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What happens to aid vision in the darkness?
Iris is dilated, photosensitive chemicals build up the rod-shaped cells on the retina, rods share nerve cells so smaller intensities can cause a signal
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How is vision in the dark different from vision in bright light?
Only the rod cells are working so we see in monochrome. Also, visual acuity is reduced so objects seem more blurred.
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In order of sensitivity, which cone cells are the most sensitive to light?
Most: Green, Red, Blue: Least
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What is meant by the persistence of vision?
The delay period between two fast moving images which appear as continuous images because rod cells take a longer time to process a signal so if another stimulus arrives, the eye cannot tell it is separate so they appear as continuous light.
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What is myopia?
Short sight
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Why does myopia occur?
The cornea is curved too much/the lens is too powerful
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What are the problems associated with myopia?
Power of the eye is too great and images of far objects are formed in front of the retina, so they appear blurred.
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What is hypermetropia?
Long sight
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How does hypermetropia occur?
The eye is not powerful enough
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How does this cause problems?
Images of near objects are formed behind the retina so they appear blurred.
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How is short sight corrected?
By using a concave (diverging) lens
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How is long sight corrected?
By using a convex (converging) lens
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What is astigmatism?
It's a condition where the cornea is not spherically curved and has different curvatures in the horizontal and vertical directions, causing an unevenly focused image on the retina
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How is astigmatism corrected?
By using a cylindrical lens
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What does a prescription for a lens to correct astigmatism need to give?
The power of the lens and the direction of the cylindrical axis
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What is the aperture of the eye?
The pupil
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What happens when circular fibres in the iris contract?
Pupil constricts
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What happens when radial fibres in the iris contract?
Pupil dilates
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What is the definition of focal length?
Distance from centre of the lens to the principal focus
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What is the relationship between the curvature of the lens and the focal length?
Higher curvature = shorter focal length
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Where are rays of light focused on the retina?
The fovea
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What is the near point of the eye?
The closest distance at which an object can be brought into focus
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What is meant by accommodation?
The ability of the lens to change its focal length
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What is presbyopia?
When the lens is stiffer making accommodation more difficult so the near point becomes further away
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What is the principal axis?
Horizontal line drawn through the centre of the lens at right angles to it.
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What sort of image does a converging lens form?
A real image
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What does it mean if the image distance is negative?
The image has been formed on the same side of the lens as the object so it is virtual.
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What is significant about the power of a diverging lens?
It is negative
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What is formula for magnification?
Image distance/object distance
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What is the name of the pigment that rod cells contain?
Rhodopsin
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What happens to rhodopsin when they absorb a photon?
They release a tiny pulse of current and are bleached
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Why is it that you temporarily cannot see when you enter a dark room after being in bright light?
Light intensity is not enough to give cone cells a stimulus and rod cells are still functioning at low levels as most of their rhodopsin is bleached.
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What is visual acuity?
The eye's ability to see in detail
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What is the distribution of cone and rod cells like on the edge of the field of view?
More rod cells, less cones cells
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And the centre of field of view?
More cone cells, less rod cells
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What is the distribution of rod and cone cells like on the fovea?
No rod cells, very high density of cone cells
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Why does the fovea have maximum visual acuity?
There are no large blood vessels and there are only radial nerve fibres, which provide an unobstructed passage for light.
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Other cards in this set

Card 2

Front

Which muscles alter the shape of the lens?

Back

Ciliary muscles

Card 3

Front

What happens when you look at an object close up?

Back

Preview of the front of card 3

Card 4

Front

What happens when you look at objects in the distance?

Back

Preview of the front of card 4

Card 5

Front

What is the main boundary at which refraction takes place?

Back

Preview of the front of card 5
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