Lesson 5 - Light, Prentice's Rule, and Abbe Values Flashcards
Visible spectrum
380 to 760 nanometers (nm). Color range: red 700 , orange 650, yellow 600, 500 green, blue 450, violet 400.
Prism diopter
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
Refraction
How light bends when passing through a medium
Law of Refraction
Light bends toward the normal when it enters a medium more dense than the one it came from.
Index of refraction
Describes the density of a material
Snell’s Law
The angle of refraction depends on the angle of incidence of the light and the indices of refraction between the two mediums
Chromatic aberration
Caused by the refraction of different wavelengths and results in colored fringes around images
Abbe value (also Nu Value or Constringent)
The amount of chromatic aberration produced by a lens material
List and describe 3 types of UV rays
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.
The protective lenses you’ll recommend in sunglasses block what?
The protective lenses you’ll recommend in sunglasses block both UVB and UVA rays.
Angle of Incidence
the angle at which the light wave hits the surface
Angle of Reflection
the angle at which light bounces off the surface
To describe a prism in a prescription, the doctor will…
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
O.D. and O.S.
O.D. is Oculus Dexter, meaning the right eye in Latin. O.S. is Oculus Sinister, meaning the left eye in Latin.
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).
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).
Sets a standard for finished lens prism tolerances.
American National Standards Institute (ANSI)
The amount of induced horizontal prism can’t be greater than a total of ___ for the two lenses combined
The amount of induced horizontal prism can’t be greater than a total of +/-.67 prism diopters for the two lenses combined
The total amount of induced vertical prism cannot be greater than ___ for the two lenses combined, or the lenses will be out of tolerance.
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.
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 ___.
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.
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 ___.
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.
What if they have too much base-in or base-out prism?
what if they have too much base-in or base-out prism? Then they’ll perceive horizontal lines as curved.
Prentice’s Rule
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.
The higher the index of refraction of a material, the…
The higher the index of refraction of a material, the more the light will bend as it goes through it.
The lower the Abbe value, the….
The lower the Abbe value, the higher the amount of light dispersion of that lens material.
List the Abbe values for plastic, glass, Trivex, and polycarbonate
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.
refractive index of air
1
