Definitions

Question 1

Sagitta

Answer 1

The distance between the highest point of a curve to the lowest point, measured over a given diameter.

Question 2

Minimum plate thickness (MPT)

Answer 2

Minimum thickness of a block that can be ordered before the surfaces are worked.

Question 3

Toric and Toroidal

Answer 3

Toric - A curved lens which has a toroidal surface.

Toroidal - A lens that has two principle radii of curvature, which corresponds to it’s two principal curves.

Question 4

Transposition - base curve, cross curve and sphere curve

Answer 4

Base curve - always lowest number, to the left.

Cross curve - always highest number, to the right.

Sphere curve - always on its own, single radius of curvature.

Question 5

Convex and concave mirrors

Answer 5

Convex:
-Diverges light
-F = minus but f = plus
- Acts like a minus lens
-Principal focus always lies between pole and the centre of curvature

Concave:
-Converges light
-F = plus but f = minus
-Acts like a plus lens
-Principal focus always lies between pole and the centre of curvature

Question 6

Thin lenses

Answer 6

Lenses where the centre substance is negligible. Total power is equal to the sum of the surface powers.

Question 7

Vergence

Answer 7

Measure of how much light is converging or diverging at any point along its path.

Question 8

Rectilinear propagation

Answer 8

The tendency of light in a straight line.

Question 9

Chief ray

Answer 9

Passes through the lens undeviated. A point where no deviation or prismatic effect occurs.

Question 10

Dispersion

Answer 10

The angle between the emergent red and blue light rays from a prism.

Question 11

Optical prisms

Answer 11

Referred to as wide-angle prisms.
Has an apical angle greater than 10 degrees (usually 40-60 degrees). Used in ophthalmic instruments.

Question 12

Refraction at a plane surface

Answer 12

When light crosses a boarder between two different refractive indices at an oblique angle, its direction of travel is changed.

Question 13

Ophthalmic prisms

Answer 13

Used to deviate light to assist the eyes of the wearer to work in harmony. Referred to as a small-angle prism as an apical angle less then 10 degrees.

Question 14

Candela

Answer 14

The luminous intensity in a given direction for emitting monochromatic radiation of frequency 540x10(sqd)Hz, the radiant intensity of which is 1/683 watts per steradian.

Question 15

Focal point

Answer 15

The point, or plane, at which light rays from infinity converge after passing through a lens and travelling a distance of one focal length.

Question 16

Differential photometry

Answer 16

The measurement of the difference in brightness of two objects.

Question 17

Absolute photometry

Answer 17

Measures very precisely the lumens emitted by a specific luminaire.

Question 18

Luminous intensity

Answer 18

The luminous flux emitted per solid angle - the power or strength of a source in a given direction.

Question 19

Luminous flux

Answer 19

The rate of flow from a source - the amount of light produced.

Question 20

Unit solid angle (steradian)

Answer 20

A way of measuring an angle in a 3D space - the angle subtended at the centre of a sphere of unit.

Question 21

Illuminance

Answer 21

The amount of illumines flux falling on a unit of a surface area.

Question 22

Back vertex

Answer 22

When the optical axis intersects the back surface.

Question 23

Front vertex

Answer 23

When the optical axis intersects the front surface.

Question 24

Back vertex power

Answer 24

Final mergence leaving the lens when the incident vergence is zero.

Question 25

Back vertex distance

Answer 25

Distance from the lens back surface to the cornea front surface.

Question 26

Critical angle

Answer 26

The demarcation between refraction and reflection where neither takes place.

Question 27

Reflection

Answer 27

Rays of light that are deflected (sent back towards their source) are reflected.

Question 28

Refraction

Answer 28

The ‘bending’ of light rays resulting from changes in the characteristics of the medium through which the rays are travelling.

Question 29

Diffraction

Answer 29

A series of waves continuing outwards from the central source to such an extent the radius of curvature is infinitely large, then the curvature of the waves is said to be zero/flat (straight line).

Question 30

Interference

Answer 30

The interaction between two or more waves.
Peak to peak = waves boosted.
Trough to peak = waves diminished.