General points

Question 1

Main Reasons for Patient Complaint

Answer 1

1- they didnt get what they thought they were getting
2- image quality/vision is poor
3- frame is not fitting comfortably

Question 2

Spherical Lenses

Answer 2

• Power is the same all around the lens, in all directions

- Used to correct myopic/hypermetropic patients only

Question 3

Toric Lenses

Answer 3

Power of cylindrical lens = no power along the axis, has 2 principal meridians at perpendicular meridians to one another
- axis direction of cylinder is always 90 to meridian where the power is positioned - axis always 90 to cyl power
-still has a max and min power but min power is not zero as it is in a spherical lens
toric is a spherical and cyl lens combined to neutralise both meridians

Question 4

Prisms

Answer 4

• Transparent block of material, has an apex and a base at the bottom which is the thickest part of the lens
• Light deviates towards the base of prism and its because it slows down as it passes through the lens
• Because light is deviated towards the base, vision and so image is deviated towards the apex

Question 5

Why is a prism used?

Answer 5

• Used for patients with muscle imbalance
• there are some pathologies which cause frequent change in vision, so surgery isnt an option
• some people are born with muscle imabalances - EOM can be too long/short so pull eye in a certain direction
• can still get px to see one image, even if both eyes see diff images

Question 6

Surfaces of a lens?

Answer 6

Front surface = away from eye

Back Surface = closest to eye

Question 7

Plano concave/ Plano convex

Answer 7

No power

Question 8

Bi concave/Bi convex

Answer 8

• same curve on front and back surface. but not necessarily of equal power
• when power is same = equi convex/equi concave

Question 9

Meniscus

Answer 9

Most common lens type = gives best optics so ensures image quality
- has one convex and one concave curve

Question 10

What is a Corrective Lens?

Answer 10

• worn in front of the eye to correct vision
• in specs its mounted in a frame to hold the lens in a short distance from the eye
• in cls it fits directly onto the surface of the cornea

Question 11

Sign Convention

Answer 11

Light always travels from left to right

Question 12

Emmetropia

Answer 12

When patient has no prescription
When parallel rays of light coming from a distant object converge to a point to form an image on the retina
Second focal point coincides with the retina
- no distortion to the image
Depends on:
- curvature of the cornea
- refractive combined power of the cornea and the lens
- length of the eyeball

Question 13

Hypermetropia

Answer 13

Point image blurry to px
Image falls after the retina and positive lens needed for image to fall on retina
Second focal point falls behind the retina
Could be due to:
- power of eyeball is too weak, so isnt focusing rays enough
- eye is a bit shorter

Question 14

Hypermetropes and Accommodation

Answer 14

Your eye can accommodate when you’re younger - lens can alter its shape to give you more/less power. I f this system is working well, they can still see objects and have good vision

Question 15

Myopia

Answer 15

Image falls before the retina and a negative lens is needed for image to fall on retina
Second focal point falls in front of the retina
- May have longer eyeballs than normal
- have v efficient system near front of the eye, so power is greater than it should be, meaning rays of light converge too much

Question 16

Astigmatism

Answer 16

Means non point forming - objects are blurred and can appear distorted
Two line foci are produced instead of one dot image
- Condition where the two optical surfaces of the eye, usually cornea , are aspherical. There are two diff curves so one will be a higher power than the other
- creates two principal powers, a max power in one meridian and a min power in the second meridian which falls perpendicular to the first

Question 17

How to correct astigmatism?

Answer 17

Lens must have two principal powers at perpendicular meridians to one another
- cylinder lens - has a surface with no power (axis meridian) which is 90 degrees away from a surface with max power (power meridian)

Question 18

Thin Lens

Answer 18

When the two surfaces are in close contact so we can ignore the refractive index of the material bet the surfaces
- power of a thin lens is the sum of the power of both the surfaces

Question 19

Thin Lens Equation

Answer 19

Fv’ = F1 + F2

Question 20

Effective Power of the Lens

Answer 20

In distance vision:
The back vertex distance in mm is the distance bet the back vertex of the lens and the cornea and it affects the power of the lens (BVD)
If you increase the distance from the lens to the eye then the lens becomes more positive - so you have to make the prescribed lens more negative