Lesson 5 - Light, Prentice's Rule, and Abbe Values

Question 1

Visible spectrum

Answer 1

380 to 760 nanometers (nm). Color range: red 700 , orange 650, yellow 600, 500 green, blue 450, violet 400.

Question 2

Prism diopter

Answer 2

A unit that describes the power of a prism to produce a 1-centimeter deviation in a ray of light at a distance of 1 meter from the lens

Question 3

Refraction

Answer 3

How light bends when passing through a medium

Question 4

Law of Refraction

Answer 4

Light bends toward the normal when it enters a medium more dense than the one it came from.

Question 5

Index of refraction

Answer 5

Describes the density of a material

Question 6

Snell’s Law

Answer 6

The angle of refraction depends on the angle of incidence of the light and the indices of refraction between the two mediums

Question 7

Chromatic aberration

Answer 7

Caused by the refraction of different wavelengths and results in colored fringes around images

Question 8

Abbe value (also Nu Value or Constringent)

Answer 8

The amount of chromatic aberration produced by a lens material

Question 9

List and describe 3 types of UV rays

Answer 9

UVC is completely absorbed by the atmosphere. UVB is the most damaging to our cells, causing cataracts and skin cancer. UVA also called black light, causes tanning and thickening of the skin.

Question 10

The protective lenses you’ll recommend in sunglasses block what?

Answer 10

The protective lenses you’ll recommend in sunglasses block both UVB and UVA rays.

Question 11

Angle of Incidence

Answer 11

the angle at which the light wave hits the surface

Question 12

Angle of Reflection

Answer 12

the angle at which light bounces off the surface

Question 13

To describe a prism in a prescription, the doctor will…

Answer 13

To describe a prism in a prescription, the doctor will write the desired amount of prism diopters and then add the delta symbol “Δ”. In addition, the doctor will write one of these abbreviations: BI (base-in: thickest part of the prism is toward the nose), BO (base-out: thickest part of the prism is toward the ear), BU (base-up: thickest part of the prism is up), BD (base-down: thickest part of the prism is down). This tells the optical lab which direction the base of the prism should face. Examples: O.D.(right) -2.00-0.25x080 .5Δ BO, O.S.(left) -2.50-1.50x090 .5Δ BO

Question 14

O.D. and O.S.

Answer 14

O.D. is Oculus Dexter, meaning the right eye in Latin. O.S. is Oculus Sinister, meaning the left eye in Latin.

Question 15

Light bends ___ as it passes through the prism—and the image a person sees moves in the ____ direction from the ___ (because the retina flips the image).

Answer 15

Light bends toward the base of a prism as it passes through the prism—and the image a person sees moves in the opposite direction from the base (because the retina flips the image).

Question 16

Sets a standard for finished lens prism tolerances.

Answer 16

American National Standards Institute (ANSI)

Question 17

The amount of induced horizontal prism can’t be greater than a total of ___ for the two lenses combined

Answer 17

The amount of induced horizontal prism can’t be greater than a total of +/-.67 prism diopters for the two lenses combined

Question 18

The total amount of induced vertical prism cannot be greater than ___ for the two lenses combined, or the lenses will be out of tolerance.

Answer 18

The total amount of induced vertical prism cannot be greater than +/-.33 prism diopters for the two lenses combined, or the lenses will be out of tolerance.

Question 19

When there’s too much base-down prism, your clients will perceive vertical objects as ____. When they’re walking straight, they’ll feel like they’re ___.

Answer 19

When there’s too much base-down prism, your clients will perceive vertical objects as taller than they are. When they’re walking straight, they’ll feel like they’re walking uphill.

Question 20

When there’s too much base-up prism, the opposite of too much base-down prism will occur. They’ll perceive vertical objects as ___, and they’ll feel like they’re ___.

Answer 20

When there’s too much base-up prism, the opposite of too much base-down prism will occur. They’ll perceive vertical objects as shorter than they really are, and they’ll feel like they’re walking downhill from a mound.

Question 21

What if they have too much base-in or base-out prism?

Answer 21

what if they have too much base-in or base-out prism? Then they’ll perceive horizontal lines as curved.

Question 22

Prentice’s Rule

Answer 22

Prism diopter (Δ) = Power of the lens (F) x the decentration distance in centimeters (dcm) between the optical center and the center of the pupil. Shortened formula -> Prism = F x dcm. Where F is the power of the lens and d is the decentration distance in centimeters.

Question 23

The higher the index of refraction of a material, the…

Answer 23

The higher the index of refraction of a material, the more the light will bend as it goes through it.

Question 24

The lower the Abbe value, the….

Answer 24

The lower the Abbe value, the higher the amount of light dispersion of that lens material.

Question 25

List the Abbe values for plastic, glass, Trivex, and polycarbonate

Answer 25

Polycarbonate: RI (refractive index value) = 1.59 and AV (Abbe value) = 30. Trivex: RI = 1.53 and AV = 45. 1.5 Glass White: RI = 1.52 and AV = 58. Standard Plastic: RI = 1.50 and AV = 58.

Question 26

refractive index of air

Answer 26

1