Dispensing

Question 1

What’s the formula to calculate the curvature of a lens?

Answer 1

R (curvature) = 1/r ( radius in meters) (units of m^-1)

Question 2

What’s the formula for calculating surface power of a lens?

Answer 2

F = (n’ - n) / r(meters) (units of D)
n is what light is going from and n’ is what the light is going into

Question 3

How to calculate focal length of a lens in (m)

Answer 3

f = n/F (meters)

In air, n = 1

Question 4

What’s the formula for spherical equivalent?

Answer 4

Sphere power + (cylindrical power / 2 )

Question 5

How do you find the position of the circle of least confusion?

Answer 5

you do the reciprocal of the spherical equivalent and the unit is m so if you want mm then you divide 1000 by the spherical equivalent

Question 6

for CR-39 what is
the refractive index
density
abbe number
reflectance as a percentage

Answer 6

1.498
1.3
58
4.0

Question 7

for crown glass what is
the refractive index
density
abbe number
reflectance as a percentage

Answer 7

1.523
2.5
59
4.3

Question 8

for polycarbonate what is
the refractive index
density
abbe number
reflectance as a percentage

Answer 8

1.586
1.2
30
5.2

Question 9

What is the formula for the curve variation factor? CVF? (this helps work out the likely change in thickness)

Answer 9

(n old - 1)/ (n new -1)

Question 10

how do you calculate transverse chromatic aberration (TCA)?

Answer 10

prismatic effect/ abbe number of the lens material
= P/V

Question 11

what is the formula for prismatic effect?

Answer 11

distance from the OC (cm) x power of the lens (D) this is prentices rule

Question 12

whats the formula for reflectance?

Answer 12

p= ((n’-n)/(n’+n))squared

Question 13

for trivex what is
the refractive index
density
abbe number
reflectance as a percentage

Answer 13

1.532
1.1
46
4.4

Question 14

whats the formula for deviation?

Answer 14

d= a(n-1) where a is the angle opposite to the right angle of the prism and d

Question 15

how do you find the prism dioptre using the apical angle?

Answer 15

P = 100 tan d

Question 16

how do you find the resultant magnitude of a prism?

Answer 16

by doing Pythagoras theorem

Question 17

what’s the formula to resolve a prims and find the horizontal and vertical components?

Answer 17

H = prism power * cos theta
V = prism power * sin theta

Question 18

how do you calculate prism at a point on a sphere?

Answer 18

P= distance from optical centre (cm) * power of the lens

Question 19

how do you calculate ocular refraction?

Answer 19

K (ocular refraction) = 1/k (far point)

Question 20

how do you calculate dioptric length of the eye?

Answer 20

K’ (dioptric length) = 1 (this number is usually always n which is 4/3) / k’ (axial length)

Question 21

whats the formula relating ocular refraction to the power of the reduced eye?

Answer 21

K (ocular refraction) + Fe (power of the reduced eye) = K’ (dioptric length of the eye)

Question 22

what’s the formula to calculate compensated power?

Answer 22

Fnew = Fold / 1+-(d x Fold)

Question 23

What is the accurate sag formula?

Answer 23

s= r - square root of r squared - h squared

Question 24

what is the approximate sag formula and when can you use it?

Answer 24

is s is small in relation to y:

r≈ y^2/2s

Question 25

what is the sag formula that relates to the power of a lens:

Answer 25

s ≈ y^2F / 2(n’-n)

Question 26

how do you work out centre thickness for
plano convex lens
plano concave lens

Answer 26

plano convex:
t=e+s

plano concave
t=e-s

Question 27

how do you work out the centre thickness of any meniscus lens?

Answer 27

t = e + s1 - s2

Question 28

what is the formula for minimum size uncut?

Answer 28

MSU = greatest distance across eye shape + 2* decentration + 2mm for wastage

Question 29

whats the formula for sag factor?

Answer 29

y^2 / 1000

Question 30

how do you calculate sag of the entire lens?

Answer 30

sag factor * power of the lens

Question 31

how do you calculate typical edge of thickness for glass or plastic?

Answer 31

e = 2.0 - (0.2 * power of the lens)

Question 32

what is the sag formula when solving for r?

Answer 32

(s squared + y squared)/ 2s

Question 33

whats the formula for CL power?

Answer 33

CL = F/1-dF where d is the BVD in meters and F is the power

Question 34

whats the formula for lens to centre of rotation?

Answer 34

vertex distance + cornea to centre of rotation

Question 35

Give a formula for thickness of the anti-reflective coating of a lens

Answer 35

t𝑓 = λ/4𝑛𝑓

-t𝑓 = the thickness of the anti-reflective coating
-lambda is the wavelength of light you want to eliminate
-nf is the refractive index of the film

Question 36

What is the formula for amplitude condition?

Answer 36

n𝑓 = √𝑛g

-nf = refractive index of film
-ng = refractive index of the lens

Question 37

What spectacle prescription would you expect for an emmetrope that has now become aphakic?

Answer 37

-spectacles: +10 to +12D
-CL: +15D
-Intraocular lens: +18 to +21D (this one is more dependent on the patient)

Question 38

what does prentice’s rule state?

Answer 38

The prism power of a lens at any point on its surface is equal to the distance away from the optical centre in centimetres times the power of the lens in dioptres

Question 39

how do you find the position of the image formed by a lens system made of two thin lenses in air separated by a distance assuming the object is distant

Answer 39

by finding the back vertex power using the equation:

F1+F2-(d.F1.F2) / 1-d.F1

Question 40

whats the equation for front vertex power?

Answer 40

F1+F2-(d.F1.F2) / 1-d.F2

Question 41

What is the formula for light transmitted to the eye through a lens?

Answer 41

p= ((n’-n)/(n’+n))^2

where n = 1

Question 42

what is the formula for sag factor?

Answer 42

y^2/1000 (this tells you the thickness per dioptre)

Question 43

What is the formula for power of the reduced eye (Fe)?

Answer 43

Fe = (4/3 - 1)/ r

Question 44

what is the standard value of power for Fe?

Answer 44

60D

Question 45

what is the standard value for axial length (k’)?

Answer 45

22.22mm