Exam 3

Question 1

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

Answer 1

0.017

w = 1/v = 1/58 = 0.017

Question 2

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

Answer 2

Abbe Value

Dispersive

Question 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?

Answer 3

20

v=.4/1.41-1.39

Question 4

Which lens materials has the highest amount of Chromatic Aberration?

Answer 4

Polycarbonate

Question 5

What has the highest Abbe value?

Answer 5

CR-39 = 58

Question 6

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

Answer 6

Prismatic

Question 7

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

Answer 7

Red

Indigo bends more

Question 8

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

Answer 8

0.3448

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

Question 9

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

Answer 9

0.3448

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

Question 10

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

Answer 10

+1.00D

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

Question 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?

Answer 11

+6.00D

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

Question 12

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

Answer 12

Reduce Chromatic Abberation

Question 13

What does not have chromatic abberation?

Answer 13

convex mirror

Question 14

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

Answer 14

Best form and Aspheric

Question 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

Answer 15

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

Question 16

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

Answer 16

True

Question 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.

Answer 17

+0.11

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

Question 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?

Answer 18

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

Question 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).

Answer 19

True

E = 1/n

Question 20

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

Answer 20

Unsymmetrical

Question 21

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

Answer 21

Point Focal

Note: Most commonly corrected

Question 22

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

Answer 22

Percival’s from lens

Question 23

Which branch is more curved? Oswalt of Wollaston

Answer 23

Wollaston

Thus leaving the Oswalt branch being less curved

Question 24

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

Answer 24

090

Pantoscopically tilt = 180

Question 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.

Answer 25

True.

Question 26

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

Answer 26

True

Question 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?

Answer 27

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

Question 28

What are the 5 seidel's sign?

Answer 28

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

Question 29

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

Answer 29

Oblique Astigmatism and Curvature of Field

Question 30

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

Answer 30

Distortion

Question 31

What can occur when a small pencil of light obliquely falls onto the periphery of a spherical spectacles lens?

Answer 31

Oblique Astigmatism

Question 32

In Oblique Astigmatism, the _______ line is further away from the lens, compared to the ________ line, which is closer to the lens.

Answer 32

Sagittal Tangential

Question 33

What has to be true in regards to Power Error in a spectacle lens, in regards to Petzval sphere?

Answer 33

Petzval is flatter (larger radius) than the Far point

Question 34

A lens design from Wollaston's branch of Tscherning's ellipse give a __________ lens.

Answer 34

Steeper

Question 35

A lens design from Oswalt branch of Tscherning's ellipse give a __________ lens.

Answer 35

Flatter

Question 36

True or False. In telescopes, 2 lenses are seperated by the sum of their focal length.

Answer 36

True. | Primary Focal Point and Secondary FOCUS of the Objective lens COINCIDE

Question 37

What is the equivalent focal power of a telescope?

Answer 37

Fe = 0

Question 38

A Keplerian telescope has _________ ocular lens

Answer 38

Postive

Question 39

A Galilean Telescope has __________ ocular lens

Answer 39

Negative

Question 40

A Galilean telescope has a +10.00D Objective lens and a -25.00D ocular lens. What is its magnification?

Answer 40

+2.5x M = -(Foc/Fob) = -(-25.00/+10) = +2.5

Question 41

What formula do you use when calculating the magnification in a telescope, in regards to diameter?

Answer 41

M = Diameter of the enterance pupil/Diameter of the exit pupil

Question 42

What formula do you use when calculating the magnification in a telescope, in regards to objective and ocular lens?

Answer 42

M = -Foc/Fob

Question 43

What is the tube length of a Kaplerian telescope with a Fob = +10.00D and Foc=+40.00D?

Answer 43

1.25cm t=(1/Fob)+(1/Foc) =1/10 + 1/40 = 4/40+1/40 = 5/40 = 0.125