Exam 3 Flashcards
A lens material has an Abbe value of 58. What is the dispersive power?
0.017
w = 1/v = 1/58 = 0.017
A lens material with a good optical quality has a high _______ and a low ________ power
Abbe Value
Dispersive
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
Which lens materials has the highest amount of Chromatic Aberration?
Polycarbonate
What has the highest Abbe value?
CR-39 = 58
Transverse Chromatic Aberration occurs when a lens expresses different ________ power value for different wavelengths.
Prismatic
Which color bends less, in regards to Transverse Chromatic Aberration?
Red
Indigo bends more
What is the LCA of a +20.00D lens with an Abbe number 58?
0.3448
LCA = F/V = +20.00/58 = 0.3448
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
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
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
What is the purpose of hybrid lenses in Low vision optics?
Reduce Chromatic Abberation
What does not have chromatic abberation?
convex mirror
What are the 2 strategies for limiting spherical aberration in spectacles lens?
Best form and Aspheric
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
True or False, In spectacle lens the best form lens must have a radii that is spherical?
True
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
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
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
Coma results in __________ blur that gets larger as an object point is moved OFF AXIS?
Unsymmetrical
A spectacle lens that is corrected for Oblique Astigmatism is called ________________
Point Focal
Note: Most commonly corrected
A spectacle lens that is corrected for the Power error is called _______________
Percival’s from lens
Which branch is more curved? Oswalt of Wollaston
Wollaston
Thus leaving the Oswalt branch being less curved
For a a frame with a face form, which degree is being tilted?
090
Pantoscopically tilt = 180
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.
True of False. The far point of a myopic eye is at a finite distance in front of the eye.
True
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
What are the 5 seidel’s sign?
Spherical Abberation, Coma, Curvature of Field, Oblique Astigmatism and Distortion
What are the 2 most important Monochromatic Aberrations when designing spectacles lenses?
Oblique Astigmatism and Curvature of Field
Which of the 5 siedel signs does not create a blurry image?
Distortion
What can occur when a small pencil of light obliquely falls onto the periphery of a spherical spectacles lens?
Oblique Astigmatism
In Oblique Astigmatism, the _______ line is further away from the lens, compared to the ________ line, which is closer to the lens.
Sagittal
Tangential
What has to be true in regards to Power Error in a spectacle lens, in regards to Petzval sphere?
Petzval is flatter (larger radius) than the Far point
A lens design from Wollaston’s branch of Tscherning’s ellipse give a __________ lens.
Steeper
A lens design from Oswalt branch of Tscherning’s ellipse give a __________ lens.
Flatter
True or False. In telescopes, 2 lenses are seperated by the sum of their focal length.
True.
Primary Focal Point and Secondary FOCUS of the Objective lens COINCIDE
What is the equivalent focal power of a telescope?
Fe = 0
A Keplerian telescope has _________ ocular lens
Postive
A Galilean Telescope has __________ ocular lens
Negative
A Galilean telescope has a +10.00D Objective lens and a -25.00D ocular lens. What is its magnification?
+2.5x
M = -(Foc/Fob) = -(-25.00/+10) = +2.5
What formula do you use when calculating the magnification in a telescope, in regards to diameter?
M = Diameter of the enterance pupil/Diameter of the exit pupil
What formula do you use when calculating the magnification in a telescope, in regards to objective and ocular lens?
M = -Foc/Fob
What is the tube length of a Kaplerian telescope with a Fob = +10.00D and Foc=+40.00D?
1.25cm
t=(1/Fob)+(1/Foc)
=1/10 + 1/40 = 4/40+1/40 = 5/40 = 0.125
