Optics Flashcards
why do we see blurry when underwater?
most of refractive power of the eye comes from the air cornea interface. if the interface were water-water, the refractive power would be less and you would only rely on the curvature of the cornea for refraction. Hence, you would be far sighted (not enough plus so acts like hyperope)
why do we see underwater physics principle
refracting power of a spherical surface formula n’-n/r tells us that in air the refracting power of the K is 52.9, but in water it is only 5.7D
what’s the spherical equivalent of -1.00+2.00x45?
sphere + 1/2 cyl= -1.00+1.00= 0 PLANO Threfore this is a jackson cross
Transpose +2.00-3.00x90 to plus cyl
1)add old sph and old cyl 2) change sign of the sphere 3) add 90 to axis -1.00+3.00x180
Loupes are an example of what type of telescope and what 2 types of lenses are used in that case?
Galiean telescope, causes upright image objective is (+) and eyepiece is (-) lens
What is the magnification of a direct ophthalmoscope?
15x uses principle of simple magnifier (Mag= D./4) The D of the ophthalmoscope is 60 Therefore 60/4= 15.
How do + and - lenses act like prisms?
+ lenses act like base down prisms - lenses act like base up prisms
Why do myopic lenses make faces seem narrower?
B/c the prismatic effect of the (-) lenses w BO (like a bowtie) will displace the virtual image of the face toward the apex
How do you compensate for small vertical deviations caused by anisometropic glasses? 3 things
1) Try contacts
2) lower both optical centers - to compromise vertical imbalance between distance and near
3) give slab off prisms
What are slab off prisms?
Used in cases of anisomeropic glasses
you measure the deviation in PD caused by the glasses, and put that prism amount on the more minus lens
what do all the dials represent?
R
O
OC
+0.50
10♦in
PH
+0.12
GL
WMHWMV
P
R- retinoscopy (puts up a +1.50D lens)
O- open
OC- occlude
+0.50- fused cross cylindar to meaure add
10♦in- 10 PD base in
PH- pinhole
+0.12- for very picky pts
GL- green lens
WMH- white maddox rod horizontal
WMV- white maddox rod vertical
P- polarized - for fly test
what does this represent?
6 PD base up OD and 9PD base out OS
where the arrow points is where the base is
In trial frames in which slot should you place the following lenses?
1) high power sphere
2) cylinder
3) low power lenses
1) high power sphere- rear cell
2) cylinder- middle cell
3) low power sphere- front cell
what is the basecurve of a lens in glasses and how can you check if it’s off?
the basecurve of a lens is different on the front and back. ex a minus lens actually has a plus front and a stronger power munis in the back. you can measure this w a geneva lens clock. sometimes a pt may not tolerate a change in the BC
what is this dial used for on a lensmeter?
if you are assessing prism in glasses and you need more prism (more than 3)
what is this and what is it used for?
placido disc-
doctors view on left, pts view on right
has a battery and rings that light up can assess k curvature grossly to assess for irrigular astigmatism
how does goldmann applinator work?
split prism 3.06mm diameter circle. when you press on the k just the right amount the circle becomes 3.06mm. why 3.06mm? its the force in dynes x10 = IOP when the circle is 3.06
whats the red line on the goldmann applinator for?
measuring iop in pts w astigmatism, you can line up the applinator with their axis of their negative cyl
whats the name of the fundus lens attached to the slit lamp and why did dr glaser like to use it?
Hruby lens. Its a 55 D lens and its best to look at ON
which is the best lens to look at the fovea?
goldmann contact lens- gives you an upright image
what are the 3 componants needed in an operating microscope?
1) binoculars
2) magnification changer (works by galiliean telescope)
3) objective lens
whats the magnification when you look at the retina using a 30D lens?
transverse mag= power of eye/power of lens
60/30=2x
what the tansverse mag of that? 2^2= 4x
what does laser stand for?
light amplification by stimulated emission of radiation
by what principle does argon laser work?
photocoagulation (local rise is tissue temperture) energy is absorbed by the tissue
by what principle does the yag laser work?
photodisruption- target is ionized and a shock wave is released
by what mechanism does an eximer laser work?
photoablation- disruption of covalent bonds-
high power ultraviolet radiation, no heat, it is a “cold “laser
whats the principle of femtosecond laser?
photoablation??
what are the 3 things you need to make a laser?
1) pulsed power to supply energy
2) active medium (photon emitter) to make light monochromatic
3) chamber w miror at opposite ends
the mirrors reflect the photons multiple time and makes the light unidirectional, the laser cavity acts like an optical resonator and 2% of the photons get transmitted through the laser
if a person sees 20/20 but still complains of poor vision what else can you test?
contrast sensitivity
how can you assess amblyopia in non-verbal child?
occlusion test
how do you check if baby has at least 20/200 vision?
okn drum-
if it induces nystagmus then baby can see at least 20/200
7 ways you can test va in pre-literate child
1) HOTV
2) Allen figures (house tree etc)
3) OKN
4) Teller cards
5) tumbling E
6) blink to light
7) vep
what is the 4 bars surrounding 1 letter for?
amblyopic kids- due to crowding phenomenon amblyopic kids can see one letter at a time better than a whole line
what is the etdrs acuity chart
1) letters of equal viewing difficulty
2) each line has 5 letters
3) spacing between the letters = letter size on that line
4) optotypes change in 0.1 log unit increments
5) 3 line change= a doubling of the visual angle
what is the equivalent of 5/30 in 20 languange (ie they can see 30 line at 5 feet?)
multiply it by 4
30x4= 120
therefore 5/30= 20/120
what are the steps in assessing for scl?
1) obtain accurate manifest refraction
2) slit lamp exm
3) keratometry
4) discuss rigid v scl
5) perform power calulations
6) fit lenses
what do you have to look for on sle when fitting scl?
L/L- anomilies, flip lids
Tears- asess tear film
Pupil- look for essentricty
K- contour, staining, neo
explain the bc of a scl?
shorter radius of the cl = steeper curve
therefore steep K will get like an 8mm
flat k will get like a 9mm
how do you determine the power of scl for a pt?
1) manifest refraction
2) if sph less than 1 then just use sph equiv otherwise needto give a toric
3) calculate the real power given vertex distance
4) if pts power is >4 then take off 0.25D for every 1D over 4 (eg if im a -6 glasses- thats 2 D over 4 so take off 0.5D off the cl)
4) check Ks- if between 42.5-44.0 then use bc 8.7
if between 44.5-46.0 then use bc 8.3
what are the 5 steps in calculating rigid lens power?
1) chose a BC +0.50 steeper than low K value
2) convert rx to minus cyl form
3) drop the cyl
4) convert to zero vertex distance
5) subtract +0.50 from final value
how do you solve vertex distance questions?
1) understand that the glasses rx is not the refactive error of the eye!
2) find FP from the glasses and add the vertex dist- that’s the true fp of the eye
3) find reciprocal of that dist to find real D error of eye
OR USE THE BACK OF THE TYLERS QUARTLY PARAMETER GUIDE IT HAS ALL THE VERTEX CORRECTIONS
what phenomenon causes the sky to be blue?
scattering (blue is scattered, the rest of the wavelengths are diffracted)
scattering means that when light hits an irregularity in its path, it scatters. Shorter wavelengths tend to scatter, and longer wavelengths (like red) tend to diffract (has ability to bend around corners)
how do you make a CL fit tighter (2 options)
1) decr BC
2) incr diameter
What do flourescein staining patterns look like if the RGP is
1) good fit
2) steep
3) flat
1) diffuse flourescein pattern
2) steep=pooling centrally
3) flat= peripheral pooling
how do you assess fit of a toric lens?
the scribe mark (ie lines) on the scl should sit at 6 oclock
if they dont use LARS acronym- (left add minus cyl, right subtract minus cyl)
how do you assess fit of a scl?
look for movement
poor movement- too tight= bc too steep
excessive movement- too flat
you prescribe a pt scl with very high minus- but the edges of the lens get causght under the lid what can you do?
bevel edges - they create a flange in the scl so it sits better
can a person w scl use solution from hard contact?
nope bc hard cl solution contains bak
person w scl sees halo at end of day what is the issue?
k edema- can cause halos and also an increase in myopia and astigmatism
scl user comes to you with chronic mucous and red itchy eye, what is the problem?
gpc
d/c the contacts use cromlyn and possibly low dose steroids
whats the srk formula?
A-2.5L-0.9K
What formula do you use for very short or long eyes?
srk2
it has a differnt a constant
the original srk formula is goog only for avg length eyes (22-24.5mm eyes)
which type of iol gives the least chromatic abberation?
one that is flat in the front and curved in the back
