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Flashcards in Exam 3 Deck (43)
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1

A lens material has an Abbe value of 58. What is the dispersive power?

0.017

w = 1/v = 1/58 = 0.017

2

A lens material with a good optical quality has a high _______ and a low ________ power

Abbe Value

Dispersive

3

By using three different wavelengths, which correspond to Fraunhofer lines d, C, and F, different refractive indices for a lens material are measured: Nd=1.40, Nc=1.39, Nf=1.41. What is the Abbe number of the lens material?

20

v=.4/1.41-1.39

4

Which lens materials has the highest amount of Chromatic Aberration?

Polycarbonate

5

What has the highest Abbe value?

CR-39 = 58

6

Transverse Chromatic Aberration occurs when a lens expresses different ________ power value for different wavelengths.

Prismatic

7

Which color bends less, in regards to Transverse Chromatic Aberration?

Red

Indigo bends more

8

What is the LCA of a +20.00D lens with an Abbe number 58?

0.3448

LCA = F/V = +20.00/58 = 0.3448

9

What is the TCA of a +20.00D lens with an Abbe number of 58, 10 mm from its center?

0.3448

P=d x F = 1cm x 20D = 20
TCA = P/V = 20/58 = 0.3448

10

A +20.00D lens has a dispersive power of 0.05. What is the LCA?

+1.00D

w=1/v = 1/0.05 = 20
LCA = F/V = 20/20 = +1.00D

11

An achromatic doublet has a focal power of +2.00D. The first lens has an Abbe Number of 60. The second lens has an Abbe number of 40. What is the focal power of the first lens?

+6.00D

F1 =(V1/V1-V2)F
60/60-40 = 60/20 = 3 x 2 = 6

12

What is the purpose of hybrid lenses in Low vision optics?

Reduce Chromatic Abberation

13

What does not have chromatic abberation?

convex mirror

14

What are the 2 strategies for limiting spherical aberration in spectacles lens?

Best form and Aspheric

15

Pick the following two answer that are correct:
1) Negative Spherical Abberation has absolute focal power and the power increases away from the lens center
2)Positive Spherical Abberation the absolute focal power increases away from the lens center
3)Paraxial image point is located closer towards the lens than non-paraxial
4)Non paraxial image point is located close toward the lens than the paraxial image point

2)Positive Spherical Abberation the absolute focal power increases away from the lens center

4)Non paraxial image point is located close toward the lens than the paraxial image point

Note: Tangential is closer = Non-paraxial image point
Sagittal is further from the image point

16

True or False, In spectacle lens the best form lens must have a radii that is spherical?

True

17

Consider image formation through a lens. The paraxial image point is 100 cm behind the lens. The non-paraxial image is 90cm behind the lens? Calculate the LSA.

+0.11

LSA = 1/ln' - 1/l'
=1/90 - 1/100 = 0.0011 =0.11D

18

True or False, A spherical lens is designed to have the least possible amount of Spherical Abberation. Therefore the front surface power must be 6 times higher than the back surface power?

True

Ratio displayed through the question is 6:1.

Note: The answer can also be 1 : -6, where the back surface power is 6 times less than the front

19

True or False. The appropriate surface eccentricity E of a spectacle lenses is calculated by the inverse of the lens refractive index (1/n).

True

E = 1/n

20

Coma results in __________ blur that gets larger as an object point is moved OFF AXIS?

Unsymmetrical

21

A spectacle lens that is corrected for Oblique Astigmatism is called ________________

Point Focal

Note: Most commonly corrected

22

A spectacle lens that is corrected for the Power error is called _______________

Percival's from lens

23

Which branch is more curved? Oswalt of Wollaston

Wollaston

Thus leaving the Oswalt branch being less curved

24

For a a frame with a face form, which degree is being tilted?

090

Pantoscopically tilt = 180

25

True or False. The far point of a hyperopic eye is at finite distance behind the eye, thus the far point of an emmetropic eye is at infinity.

True.

26

True of False. The far point of a myopic eye is at a finite distance in front of the eye.

True

27

What is the correct answer.
Consider a myopic pt., corrected with a spectacle lens. The lens is 12mm from the eye (d), has a secondary focal length of -20cm. Where is the far point?

+21.20

r=d-f
1.20 - (-20) = +21.20

28

What are the 5 seidel's sign?

Spherical Abberation, Coma, Curvature of Field, Oblique Astigmatism and Distortion

29

What are the 2 most important Monochromatic Aberrations when designing spectacles lenses?

Oblique Astigmatism and Curvature of Field

30

Which of the 5 siedel signs does not create a blurry image?

Distortion