L2 Prescription and Lenses

Question 1

What is a Corrective Lens?

Answer 1

• 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 2

Sign Convention

Answer 2

Light always travels from left to right

Question 3

Focal Length

Answer 3

From the vertex of the lens to the 2nd principle focal point

• a lens with a relatively shorter focal length is a more powerful stronger lens, a steeper curved lens
• a lens with a relatively longer focal length is less powerful, weaker lens, a flatter curved lens

Question 4

Dioptre

Answer 4

Power of a lens in dioptres is the reciprocal of its focal length in metres

• measured in 0.25 steps
• can represent the power of sphere or cylinder
• must be written with a +/- sign and must be to 2dp

Question 5

Emmetropia

Answer 5

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 6

Axial Length of Eyeball

Answer 6

Generally around 24mm

Question 7

Hypermetropia

Answer 7

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 8

Hypermetropes and Accommodation

Answer 8

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 9

Myopia

Answer 9

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 10

Astigmatism

Answer 10

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 11

How to correct astigmatism?

Answer 11

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 12

Thin Lens

Answer 12

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 13

Thin Lens Equation

Answer 13

Fv’ = F1 + F2