Dispensing - power and lenses

What can be used to work out the power of a lens surface?

The radius of curvature

How do we know what measurements we are working in?

• 1/*** → m

• 100/*** → cm

• 1000/*** → mm

Equation for R?

R = 1000/r

What is the equation for F1?

F1 = (1000(n-1) / r1 mm)

What is the equation for F2?

F2 = (1000(1-n) / r2 mm)

How do we MNOW if we should be doing n-1 or vice versa?

• it’s the refractive index of the medium which the light is about to enter minus the reflective index of the medium where the light has left

What is the lens makers equation?

Combo of the two previous equations

F = (n-1)(1/r1-1/r2)

Can instead work out F1 and F2 separately then add tgth

What are thin lenses?

• when two surfaces are in close contact with one another

• Can ignore the refractive index of the material in between

• Sum of the power if both surfaces gives the overall power of the thin lens

• F = F1 + F2

What do we do if none of the previous measurements are given to work out surface power?

Use the fundamental paraxial equation

What is the fundamental paraxial equation?

• L’ = L + F

• L → incident vergence

• L’ → exit vergence

• F → optical power of the surface

How to calculate L and L’?

• L = n/l

• L’ = n/l’

Define back vertex power

Vergence leaving the back of the lens when the vergence arriving at the front surface is 0

Define front vergence

• vergence leaving the front of the lens when the vergence incident at the back surface is 0

Are focal lengths additive?

• no, only lens powers

What must we always convert focal lengths to?

Dioptres

When light is at infinity what is L always?

0

In a step along, what do all the letter represent?

How would you work out the back vergence in this question?

What is back vertex distance?

• Back vertex distance (BVD) is the distance from the front surface of the cornea to the back side of the a lens that is mounted in a frame and being worn by the patient

What is BVD measured in ?

mm

Why does BVD matter?

• if you increase the distance from the lens to the eye then the lens becimes more positive, therefore you have to make the prescribed lens more negative

How can we work this question out adjusting for BVD?

When light travels through a medium other than air, what do we do?

• The real thickness of the material (t) can be replaced by an equivalent thickness of air (d)

• d = t/n