Week 1.04 Surface Power, Lens Measure And Thick Lens Theory Flashcards
Sign convention
Right = positive
Left = negative
Up = positive
Down = negative
Clockwise angles = negative
Anticlockwise = positive
Snells law
N x sin1 = N’ x sin2
What direction does light travel in
Left to right
The formula for refractive index of a medium
Refractive index of a medium = velocity of light in vacuum / velocity of light in medium
What is the front surface formula
F=(n’-n)/r
(Remember new medium minus the old medium)
Radius in metres
What is the exact sag formula
S = r - √(r^2 - y^2)
How do you rearrange the sag formula to workout the radius
R = (y^2 + s^2) / 2s
What’s the refractive index of ophthalmic crown glass
1.523
What is the actual surface power of a lens made from flint glass (n=1.654) when the lens measure displays a surface power of +6.00D
Use the formula:
Ftrue = Fmeasured x ((ntrue -1) / 0.523)
Ftrue = +7.50D (check notes 1.04 if unsure)
What is the refractive index of a lens when a lens measure calibrated for crown glass (n=1.523) displays a surface power of +6.00 but the actual surface power is +7.50D?
Use the formula:
ntrue = 1+ ((Ftrue/Fmeasured) x0.523)
Ntrue = 1.654 (check notes 1.04 if unsure)
What is the alternative way of writing the true surface power formula
True surface power = (n-1)/(1.523 - 1) x lens measure reading
What is the back vertex power formula
BVP = F1/ 1 -(t/n xF1) + F2
T in mm
What is the front vertex power formula
FVP = F2 / 1 - (t/n x F1) + F2
T in mm
What will increasing the centre thickness of either positive or negative lens do to the BVP
Will make the BVP more positive if all other parameters stay the same
