Equations

Question 1

snells law

Answer 1

n(sini i)= n(sin i’)

apparent depth can appear to be smaller than the real depth when n is more than 1

Question 2

distance equivalent air thickness

Answer 2

d=t/n

d= equicalent air thickness
t= real thickness. just in mm

Question 3

power of a lens

Answer 3

F=1/f’
f’ is in m

F= F1+F2

bvp and fvp distance is in m. d=t/n
thickness is in m as well

Question 4

lens curvature equation

Answer 4

F= n’-n/r
radius is in m

remember n’-1 front surfac and if back surface 1-ans bc were in the lens

Question 5

sag

Answer 5

exact sag is s=r square root r squared- y squared
radius and sag and y in mm

approx sag is y squared/2r again all in mm

and then theres approx sag but w refractive index and power approx.

Question 6

centre thickness equation

Answer 6

tc= te+s1-s2

tc and te is mm. S1 and s2 is front surface back surface and mm.

Question 7

lens measure thickness

Answer 7

lens measure measures the thickness by applying formula but it is calibrated for glass

FT=FLM times (n’-1)/0.523

Ft= true surface power
flm= surface power from lens measure

Question 8

target x

Answer 8

x= (1/F0 squared) 1000
newtons relationship

x= movement of the target per dioptre
F0= standard lens power

times by cyl

Question 9

lens power w spectacle lenses

Answer 9

F= k/1+dk

k= ocular refraction
F= spectacle lens power
d= vertex distance of spectacle lens (+ve)

Question 10

for the decentration long equationn

Answer 10

x= horizontal decentration in cm
y= vertical decentration in cm
s= spherical power
c=cylindrical power
theta= cylinder axis

positive or negative decentration determines the base

if the theta is for the left or right lens.
left lens= theta is cyl axis
right lens- theta is 180-cyl axis

Question 11

minimum sized uncut equation

Answer 11

msu= (hcd-pd) +LD +2mm

msu= minimum sized uncut
hcd= horizontal centre distance
ld= lens diameter
2mm is for the bevel

hcd= horizontal lens size+distance between lenses

Question 12

prentices formula

Answer 12

c=p/f

c= distance from optical centre so decentration in cm

make sure you decentre first if you havent for this question

apply sign convention for the p and also just work out the base direction by sign convention as well

Question 13

retinal image size equation

Answer 13

h’=x tan theta

h’= retinal image size
x= distance from target = always 16.67

Question 14

spectacle magnification

Answer 14

corrected image size/uncorrected image size

Question 15

rsm equation

Answer 15

corrected ametropic image size/ ememtropic image size

Question 16

cvf

Answer 16

on the formula sheet
if we get 0.65 1.00-ans= 0.35 = 35 percent thinner

Question 17

transmission and tint

Answer 17

both re arranged equations are on sheet
twice density need twice the tint

work out tint density for it then / by it to get it in 1mm
thenwork out transmission

Question 18

coating needs to have single ar

Answer 18

square root n of lens= coating refractive index

coating must have a n of the square root of n of lens

Question 19

reflectance formula

Answer 19

on formula sheet
work out for front surface to see how much has been reflected then do 100-ans to see how much light is left=how much light was acc transmitted through

then do this ans times by p again because that same amount of light will be reflected again

add what is lost for both surfaces for p.