Handout 3

Question 1

The angle though which the surface turns in a unit length of arc

Answer 1

Curvature (R)

Question 2

The smaller the radius the ___ the curvature

Answer 2

Larger

Question 3

The larger the radius the __ the curvature

Answer 3

Smaller

Question 4

Units for curvature

Answer 4

Diopters/ reciprocal meters

Question 5

The distance between a point on a circle and midpoint of a chord of a circle

Answer 5

Sagitta, sag, s

Question 6

A geometric line segment whose endpoints both lie on the circle

Answer 6

Chord

Question 7

When chord diameter is kept constant and curvature is decreased / radius is increased

Answer 7

Lens power and sag decrease

Question 8

Sag is associated with

Answer 8

Power of the surface

Question 9

Lens clock AKA __, measures

Answer 9

Lens measure/ lens gauge aka Geneva lens measure

• measures surface curvature using sagitta
• chord diameter :constant

Question 10

Smaller index of refraction than the lens clock will give you

Answer 10

A weaker RX

Question 11

Greater index than lens clock will give u

Answer 11

Stronger RX

Question 12

Refers to lens shape due to front and back surface curvatures of lens

Answer 12

Lens form

Question 13

Spherical lens forms include

Answer 13

Plano- convex 
Plano- concave 
Biconvave 
Bioconvex
Modern: bent or meniscus lenses

Question 14

Lens form used in spectacle lenses

Answer 14

Bent or meniscus

Question 15

Non spherical lenses include

Answer 15

Cylindrical, toric/spherocylindrical

Question 16

Cylindrical lens is

Answer 16

A cylinder cut in half describes by 2 perpendicular axis

• power axis= cylinder meridian
• cylinder axis =power meridian

Question 17

Meridian that has 0 power

Answer 17

Axis meridian

Question 18

Axis position of lenses is in a ___ notation

Answer 18

Degrees

Counterclockwise

Question 19

Lens form for patient who has myopia or hyperopia with astigmatism

Answer 19

Toric, spherocylindrical

Question 20

The lens curve on the surface of a lens, from which the other curves are determined

Answer 20

Base curve/ BC

Always on the front surface of single vision lenses

Question 21

BC of a segmented multi focal is always on what side?

Answer 21

Same side of lens as the segment

Question 22

We measure base curve with what?

Answer 22

Lens clock

Question 23

Careful selection of base curves by the manufacturer minimizes

Answer 23

Monochromatic aberrations/ distortions

Question 24

Lenses designed to minimize these aberrations are called

Answer 24

Corrected curve or best form

Question 25

Why optometrists need to know the base curve?

Answer 25

• affects how shapes and straight lines appear
• change the way peripheral objects are seen
• some are sensitive to changes in BC

Question 26

When to consider measuring BC?

Answer 26

• Replacement of only one lens

- ordering identically powered second pair