Optics

Question 1

• Lensmaker equation

Answer 1

P=(n2-n1)/r

Question 2

• Angle kappa

Answer 2

angle between pupillary axis and visual axis, very important in refractive surgery since machines tend to use pupillary center

Question 3

• Angle alpha:

Answer 3

angle between optical axis and visual axis. Very important in cataract surgery. Esp with multifocal lens.

Question 4

• Angle gamma

Answer 4

angle formed between fixation axis and visual axis

Question 5

• Optical axis

Answer 5

optical center of cornea to optical center of lens

Question 6

• Pupillary axis

Answer 6

midpoint of pupil

Question 7

• Visual axis

Answer 7

visualized target to center of fovea

Question 8

• Transverse magnification of any telescope

Answer 8

Pe / Po

Question 9

• Bifocals

Answer 9

round top have optical center at the bottom—resulting in maximal image jump. Flat top has optical center near the top, so less image jump. For + lenese, round top has less image displacement, and for – lenses, flat top has less image displacement. Displacement is more bothersome than image jump.

Question 10

• Pantoscopic tilt:

Answer 10

if you tilt a – lens on the 180 degree axis it will induce cylinder in the 180 degree meridian that works at the 90 degree axis. If the lens is + the change in cyl will be +, if the lens is – then the change in cyl will be -. So tilt a – lens at 180 degree axis and the lens will be more – and there will be an induction of negative cylinder in the 180 degree axis.

Question 11

• Switching from glasses to contacts

Answer 11

Myopes will need More accommodation, hyperopes will need less accommodation.

Question 12

• Convex Mirrors

Answer 12

a convex mirror can only form virtual images on the opposite side of the object. The power is ALWAYS negative.

Question 13

• Prisms for optics

Answer 13

real light rays bend toward the base, virtual to the apex. Light entering the eye is real so bent towards the base. Light coming from the eye (what you see through the prism) is virtual, so it will look bent towards the apex. Also think if light bends towards the base, think in a base down prism, light through prism bends down hits inferior retina and brain will interpret that as coming from superior so it will look like light moves up.

Question 14

Refracting power of a spherical surface (in diopters)

Answer 14

Ds = (n’ - n) / r Where D is the power in diopters of the sphere N’ is the index of refraction of the spherical surface and n is the index of refraction of the external medium. (Like if air then n’-1) and r is the radius of curvature of the sphere

Question 15

Calculating power for thin Lens when changing surrounding medium

Answer 15

Dair/ D new medium = (n lens - n air) / (n lens - n new medium) (Last minute optics page 12)

Question 16

Schematic eye to find sizes of objects in real life compared to object size on retina

Answer 16

Two like triangles so you can use a ratio. If you have the object distance you can compre to the 17 mm distance from nodal point to retina to calculate relative size. Object height / retinal image height = object distance from nodal point / 17 mm

Question 17

Correcting for refractive lanes

Answer 17

Refraction lanes are less than infinitely long so patients are left 0.167 D under corrected. You can add a little minus (-0.25 D) to compensate for this.

Question 18

Problems with high powered plus spectacles

Answer 18

Ring scotoma with high plus spectacles Pincushion distortion (periphery of image is magnified more than the center) Excessive magnification Weight and cost of lenses

Question 19

Why would a hyperopic man slide his glasses down his nose?

Answer 19

Moving a lens whether plus or minus away from the eye gives it MORE effective PLUS power. If the plus lens is too weak, sliding the glasses further away will make the lens effectively more powerful. (This man is under corrected.) In a myope it also induces more plus power so patient in myope could be over corrected or sliding the glasses down to help with near work where the extra plus power is helpful.

Question 20

Changing lens position for glasses or contacts

Answer 20

Calculate the far point of the eye based on diverging or converging rays (for myopes it will be in front of Telehealth eye for hyperopes behind the eye). Then just take 1 over the length of the far point to calculate the new power. Plus lenses closer to the eye will have to be more powerful and further out from the eye less powerful (compare IOL powers to aphakic glasses to help you remember.) myopic lenses will also need to be MORE PLUS closer to the eye (so less minus). A myope will need a weaker correction in contacts than glasses, and a hyperope will need stronger correction.

Question 21

Accommodation: near point

Answer 21

Near point is the point of clearest vision (conjugate to the retina) when accommodation is maximally active.

Question 22

Accommodation: far point

Answer 22

Far point is the point that is conjugate to the retina when accommodation is fully relaxed. A myopic Eye has a far point somewhere between infinity and the cornea whereas a hyperopic Eye has a far point somewhere behind the eye.

Question 23

Absolute hyperopia

Answer 23

Without cycloplegia the least amount of plus correction required for clear vision at distance

Question 24

Manifest hyperopia

Answer 24

Without cycloplegia the most plus correction the eye can accept without blurring of vision

Question 25

Facultative hyperopia

Answer 25

The difference between the absolute and manifest hyperopia

Question 26

Latent hyperopia

Answer 26

The difference between manifest hyperopia and the hyperopia measures with cycloplegia

Question 27

Accommodation with contact lenses

Answer 27

A myope will have to accommodate more with contact lenses than spectacles. A hyperope will have to accommodate less. Be careful of putting a middle aged myope into contacts for the first time!

Question 28

Focal length formula

Answer 28

1/D

Question 29

Vergence formula

Answer 29

U + P = V

Question 30

Prentice’s rule: formula

Answer 30

PD = hD (PD is prism diopters, h is distance from optical center in cm, D is diopters of lens power.)

Question 31

Spherical equivalent

Answer 31

SE = Sph + 1/2cyl

Question 32

Retinal image height formula

Answer 32

Object height / retinal image height = distance from nodal point / 17 mm

Question 33

Spectacle lens magnification formula

Answer 33

M spec = 2% per diopter of power (assuming 12 mm vertex)

Question 34

Transverse magnification formula

Answer 34

M = i/ o Or M = U / V

Question 35

Indirect ophthalmoscope lens formula

Answer 35

M indirect = Deye / Dlens = 60/Dlens

Question 36

Axial magnification formula

Answer 36

Maxial = M^2 transverse

Question 37

Simple magnifier formula

Answer 37

M simple magnifier = D / 4

Question 38

Telescope magnification

Answer 38

M telescope = D eyepiece / D objective

Question 39

SRK formula

Answer 39

D IOL = A -2.5 (L) - 0.9 (K) Where L is axial length in mm, D is recommended power for emmetropia and K is keratometry readings only diopters

Question 40

Reflecting power of a spherical mirror (formula)

Answer 40

D reflecting = 1/f = 2/r