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Opt 127: Ophthalmic Optics > Properties Of Lens Materials > Flashcards

Properties Of Lens Materials Flashcards

0
Q

Index of refraction (n)

A

Speed of light in a vacuum compared to its speed in an optical material

velocity of light in vacuum / velocity of light in material

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1
Q

What is a lens?

A

Transparent material between two polished surfaces

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2
Q

What wavelength is yellow helium line?

A

587.6 nm

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3
Q

What is considered normal index?

A

> 1.48 but < 1.54

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4
Q

What is dispersion (dispersive power)?

A

Chromatic aberration of an optical material

Spread of white light as it’s passed through material

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5
Q

Equation for dispersion (w)

A

(w) = (nf - nc) / (nd -1)

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6
Q

Blue light

A

nf

486 nam

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7
Q

Red light

A

nc

656 nm

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8
Q

High dispersion =

A

= High chromatic aberration

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9
Q

What is the Abbé number?

A

Constringence
V = 1 / w
Usually around 30-60

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10
Q

What is axial/longitudinal chromatic aberration?

A

Aberration occurs on the optical axis

Inherent in lens material

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11
Q

Equation for axial chromatic aberration

A

ACA = F / V

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12
Q

What is transverse/lateral chromatic aberration?

A

Measures differences in chromatic aberration via prismatic differences

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13
Q

Equation for transverse chromatic aberration

A

TCA = (Fd) / V

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14
Q

What is specific gravity?

A

Weight of a given volume of material compared to the weight of the same volume of water

Depends on density

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15
Q

Higher specific gravity =

A

= heavier material

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16
Q

Glass =

A

= Mineral

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17
Q

What material is in glass?

A

Lead

Higher index -> heavier glass

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18
Q

What is glass?

A

Amorphous, supercooled liquid with high viscosity and no discrete melting point

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19
Q

What are the basic ingredients for crown glass?

A 
Silica (sand)
Sodium oxide (soda)
Calcium oxide (lime)
20
Q

What is ophthalmic crown glass made of?

A

Potassium
Borax
Antimony
Arsenic

21
Q

What is absorptive glass?

A

Non-visible radiation absorption

Photochromatic

22
Q

What makes photochromatic lenses change from light to dark?

A

Silver halide crystals

23
Q

Which glass material is the most optically superior and stable?

A

Crown glass

24
Q

Reasons why crown glass is the best glass:

A 
Lowest chromatic aberration 
High Abbé number
Least warpage
Good optics
Difficult to scratch

High specific gravity and low refractive index

25
Q

What is the refractive index of crown glass?

A

1.523

26
Q

What is the Abbé number of crown glass?

A

59

27
Q

What is the specific gravity of crown glass?

A

2.54

28
Q

Plastic =

A

= Organic

29
Q

What is plastic?

A

Polymeric material of large molecular weight

Synthetic material formed by combining organic with inorganic materials

30
Q

What are the two main groups of plastic material?

A

Thermoplastic

Thermosetting

31
Q

What is an example of thermoplastic material?

A

Polycarbonate

32
Q

What is an example of thermosetting plastic material?

A

CR-39

33
Q

What is thermoplastic material?

A

Softens when heated
Can be stretched, pressed, or molded
Plastic memory

Not stable, susceptible to heat deformation

34
Q

Explain the thermosetting process.

A

Granules of raw material are melted.

The molten material is injected into mold.

35
Q

What are thermosetting materials?

A

Once hardened, can’t be softened

Stable, rigid, flame resistant, insensitive to heat

36
Q

Explain the thermosetting process.

A

Liquid monomer is mixed with a catalyst and placed in molds then cured.

37
Q

What does CR-39 stand for?

A

Colombian Resin, 39th lab formation

38
Q

How did CR-39 come about?

A

At the end of WWII, Pittsburg Plate Glass had a railroad tank full of CR-39 monomer without a buyer.

39
Q

The original CR-39

A

Slightly yellowish tinge

Soft surface, easily scratched

40
Q

Characteristics of CR-39:

A

Better impact-resistant than untreated

Lower specific gravity than glass

41
Q

Characteristics of polycarbonate:

A

Greatest impact resistance
Higher refractive index, low specific gravity,

More dispersion than glass

42
Q

What material has the best impact resistance?

A

Polycarbonate

43
Q

What is the refractive index for CR-39?

A

1.499

44
Q

What is the Abbé number for CR-39?

A

58

45
Q

What is the specific gravity for CR-39?

A

1.32

46
Q

What is the refractive index for polycarbonate?

A

1.586

47
Q

What is the Abbé number for polycarbonate?

A

30

48
Q

What is the specific gravity for polycarbonate?

A

1.20

Opt 127: Ophthalmic Optics (2 decks)

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