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The index of refraction of any given material is calculated by __
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The index of refraction of air is __
1.000
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CR-39 plastic has an index of refraction of 1.498. This means that __
it travels 1.498 times slower than it travels in air.
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the standard index of refraction used in the optical industry. Most optical equipment and tools are calibrated to this index.
1.530
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3 things that can affect the power of a lens
- index of refraction of the material
- thickness of the lens
- relationship between the front and back of the lens
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When light hits a lens, 3 things happen:
ex. uncoated CR-39 (crown glass): About 4% is reflected from the outer & inner surface. Aprox 1% will be absorbed. Total transmission is 91%
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Light is always bent toward the __ of a prism
base
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A prism has the ability to __ images.
displace
When a ray of light which has been refracted toward the base of the prism enters the eye, it is perceived as having been traveling in a straight line, even though in reality it has been bent. Therefore, the image of the original object appears to be in a position other than the position from which the light had originally emanated.
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If the original object is 1 meter from the prism, and the image is displaced a distance of 1 cm, that prism is said to have the power of __
1 prism diopter (1 cm/1 m = 1D)
"O" indicated the original position. "I" indicates the location of the displaced image.
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When 2 prisms are placed base to base:
- They form a plus lens.
- Plus lenses cause parallel rays of light to converge or come together forming real images.
- The distance from the lens to the focal point is called the focal length.
ex. The focal length of a lens is = to 1 meter, the lens power is 1 diopter. Since it is converging, it is a plus lens or +1.00 diopter.
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When 2 prisms are placed apex to apex:
- They form a minus lens.
- Minus lenses cause parallel rays of light to diverge.
- Extending the rays backward would eventually meet at a virtual focal point forming a virtual image.
- The distance from the lens to the virtual focal point is called the focal length.
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The stronger the power, the __ the focal length. The weaker the power, the __ the focal length.
shorter; longer
formula: D (diopter power) = 1/f (focal length in m). f = 1/D
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A plus lens has a focal length __ the lens and a minus has a focal length __ the lens.
behind;in front of
The focal points of minus lenses fall in the negative quadrant of the x axis while those of plus lenses fall in the positive quadrant.
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A prism is present everywhere except for the point at which __ in a minus lens and __ in a plus lens
- the apices meet - the optical center;
- the prism bases meet - the optical center
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If no prism is prescribed, the optical centers should be positioned __
directly in front of the pupils (as in figure B)
When the optical center is positioned at some other point, prism, either wanted or unwanted, is induced.
The direction of the base will depend upon where the OC is positioned relative to the pupil.
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minus lens prism vs plus lens prism
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For a patient with no prescribed prism, the __ of a single vision lens is the optical center.
major reference point (MPR)
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Prentice's Rule
determines prism power
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How much prism is induced 4 mm away from the optical center of a +3.00 D spherical lens?
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The total of 2 diopters of prescribed vertical prism is present. 1.0 Δ base up in the right eye combined with 1 Δ base down in the left eye. It is possible to redistribute the prism between 2 lenses without alternating its effective power. This is best done by:
- In the 1st example, additional thickness & weight contributed by the prism is evenly distributed or split between the 2 lenses.
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