THE EYE AS A COMPOUND LENS Flashcards
1
Q
- What is the Gullstrand Exact eye?
A
- it is a schematic model for the imaging rays in the eye
- it is the most complete model
- it has six refractive surfaces
- there is a variation of the refractive index within the
lens
2
Q
- What is the Reduced Eye?
A
- this is the simplest model
- it only has one refractive interface at the cornea
- all the distances are relative to the single refractive
interface
3
Q
- What is the Schematic Eye 1 Model?
A
- the refractive index within the crystalline lens is
uniform - it shows the surfaces of the four interfaces
- it shows the refractive indices of the five media
- it shows the radii of the four surfaces
4
Q
- How is the refractive power of each interface given?
A
- ignore the distances that the rays propagate
between the refractive interfaces
5
Q
- Does this working out make sense?
A
- yes
6
Q
- Does this working out make sense?
A
- yes
7
Q
- What are two thirds of the Refractive Power due to?
A
- the cornea
8
Q
- What is one third of the Refractive Power due to?
A
- the lens
9
Q
- How do we work out the Total Refractive Power of the eyes?
A
10
Q
- How is the focal length found?
A
11
Q
- Does this make sense?
A
- yes
12
Q
- What happens when you swim in water?
A
- there is a change in the refractive index
- it is changed from air to water
- this changes the refractive power of the first interface
- there is now a lower refractive power
13
Q
- What can be said about the Refractive Power of the Cornea under water?
A
- the refractive power of the Cornea is:
5.19 D + (-6.11D) = -0.2 D - this means that there is essentially no refractive power
under water
14
Q
- What kind of images do the eyes see underwater and why?
A
- the total refractive power of the eye is:
-0.2 D + 22.01D = 21.90 D - the eye has refractive power
- the eye sees very blurred images
- this is because the focused image would lie beyond the
retina
15
Q
- Why can we see much better under water with goggles?
A
- the refractive power changes
- this is through the use of plastic
- the images in the water are made even blurrier when
the water is not perfectly still - this is because the movement of water causes local
variations in the index of refraction
16
Q
- What is Accomodation?
A
- the ability to control the focal length of the Crystalline
lens
17
Q
- What is the Crystalline Lens suspended to?
A
- it is suspended by ligaments
18
Q
- What do the ligaments of the Crystalline Lens control?
A
- the tension in these ligaments controls the curvature
of the crystalline lens surfaces - this then controls the focal length
19
Q
- What happens when the tension in the ligaments is at a maximum?
A
- the crystalline lens flattens
- the focal length is at a maximum
- a flatter lens will produce clear images of distant
objects - this is proven by the Lens Equation
20
Q
- What happens when the ligaments are relaxed?
A
- the crystalline lens assumes its more normal and
spherical shape - a larger curvature of the crystalline lens surface
produces a shorter focal length - this produces a clear image of nearby objects
21
Q
- What is the Far Point?
A
- this is the maximum distance from the eye to the
objects - where the objects form sharp images
- we would like our Far Point (FP) to be infinite
22
Q
- What is our Near Point?
A
- this is the nearest distance where objects are clear
- we would like our Near Point (NP) to be 25cm
- this is suitable for reading
23
Q
- What is Presbyopia?
A
- it is the lack of appropriate accommodation
- this means old ages
- or old age vision
24
Q
- What are three causes of loss of Accomodation?
A
- an increase in the Crystalline lens’ Young’s Modulus
- a flattening of the lens
- decrease in the Lens’ Capsule Young’s Modulus
25
Q
- What is Hyperopia?
A
- this is farsightedness
- this is when it is difficult for a person to see things up
close
26
Q
- What is Hyperopia caused by?
A
- the focal length is too long
- the image is focused behind the retina
27
Q
- What helps with Hyperopia?
A
- a convex lens
- the convex lens helps the eye refract the light
- this decreases the image distance
- this decreases the focal length
- the image is focused on the retina
28
Q
- Who does Hyperopia usually affect?
A
- it usually affects adults with weakened ciliary muscles
- or adults with decreases lens flexibility
29
Q
- What is a good way to remember Hyperopia?
A
- FAR SIGHTED
- YOU CAN SEE FAR
- THE RAYS FOCUS TOO FAR FROM THE LENS
30
Q
- What is Myopia?
A
- it is when someone is near sighted
- the focal length is too short
- this causes the images of distant objects to be focused
in front of the retina
31
Q
- What helps with Myopia?
A
- a concave lens
- the concave lens diverges the light rays
- this increases the image distance
- this is so that the image can be focused on the Retina
32
Q
- Who is nearsightedness common among?
A
- it is common among young people
- this is sometimes the result of a bulging cornea
- this will refract light more than normal
- it can also be the result of an elongated eyeball
33
Q
- What is a good way to remember Myopia?
A
- NEARSIGHTEDNESS
- MEANS YOU CAN SEE NEAR
- THE RAYS FOCUS TOO NEAR TO THE LENS
34
Q
- Does this summary make sense?
A
- yes
