OL 3

Question 1

Define curvature

Answer 1

Curvature is the measurement of the shape of a curved surface:

the angle through which the surface turns in unit length of arc

curvature is not dependent on the material of the surface

Question 2

give equation for curvature

Answer 2

R = 1/r

Question 3

what should be considered regarding lens thickness?

Answer 3

The thickness of a spectacle lens should be sufficiently robust, whilst being as thin as possible to maintain a good cosmetic appearance

Question 4

when must you use the approximate form of the sag equation?

Answer 4

For surfaces of small diameter (up to approx 60 mm) and long radii (up to approx ± 11.00 D) of curvature the exact sag equation may be simplified to an approximate form : s=y^2 /2r

Question 5

give the equation for the following positive lenses thickness:

1) biconvex
2) plano-convex
3) convex meniscus

Answer 5

Biconvex: tc =s1+s2 +te
Plano-convex: tc =s+te
convex meniscus: tc =(s1 +te)-s2

Question 6

give the equation for the following negative lenses thickness:

1) biconcave
2) plano-concave
3) concave meniscus

Answer 6

biconcave: tc =te –(s1 +s2)
plano-concave: tc =te –s
concave meniscus: tc =(te –s2)+s1

Question 7

what does the lens measure do?

Answer 7

The lens measure is a practical application of the exact sag equation

Question 8

in order for the lens measure to work, what must be assumed?

Answer 8

n’

lens measures are calibrated for optical crown glass, thus n’= 1.523

Question 9

what is the limitation on the lens measure equation?

Answer 9

We want to use a lens measure on a material other than crown glass of known refractive index