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Lens Characteristics Flashcards

1
Q

What is a prism?

A

a triangular shape with an apex and base

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2
Q

What is the apex?

A

It is the point at the top of a prism.

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3
Q

What is the base?

A

It is the wider, flat surface at the bottom of a prism.

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4
Q

What happen to an object when viewed through a prism?

A

It is moved in a different direction as compared to seeing the object with the naked eye.

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5
Q

What happens when light passes through a prism?

A

it is redirected toward the base of the prism.

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6
Q

What makes the object seen by the patient through a prism appear to be displaced?

A

Toward the apex.

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7
Q

What is the ratio used to predict image displacement?

A

P=C/D
C=displacement of the object in cm.
D=distance from the prism in meters

ex:What is the power of a prism that displaces light 1cm at a distance of 0.5 meter?
P=1cm/0.5m
P=2.00D
Therefore the power of the prism 2(triangle) diopters.

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8
Q

Lenses are a combination of prisms. Plus lenses are prisms joined …?

A

Base to base.

that is why plus lenses are thicker in the middle and thin on the edges.

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9
Q

Minus lenses are prisms joined…?

A

apex to apex.

Thin in the middle and thick on the edges.

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10
Q

What is a focal length or point of focus?

A

every lens has this and is defined as the distance a lens takes to focus parallel beams of light.

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11
Q

Stronger lenses have ….?

A

short focal lengths

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12
Q

Weaker lenses have…?

A

longer focal lengths

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13
Q

What is a lensometer?

A

it measures the focal length of a lens and converts it into diopters, which is the unit of measure used for writing Rx.

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14
Q

What is the formula used to calculate the focal length?

A

f=1/F
f=focal length
F=lens power in diopters
D=distance from prism in meters

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15
Q

Describe the focal length of a plus lens.

A

having a “real” focal length because the base to base prisms converge light to a measurable point of focus behind the lens.

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16
Q

Describe the focal length of a minus lens.

A

has a “virtual” focal length because apex to apex prisms diverge light rays that never come to a point of focus.

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17
Q

What is a virtual focal point?

A

Their paths must be reconstructed backward to determine focal point..

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18
Q

EX: What is the focal length of a +2.00D lens?

A

f=1/2 f=0.5m real

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19
Q

What is the focal length of a -4.00D lens?

A

f=1/4 f=0.25m virtual

20
Q

What is the power of a lens with a virtual 2m focal length?

A

F=1/2 F=-0.50

21
Q

What is the optical center?

A

The single point on the lens through which light passé without being deviated or refracted.

22
Q

In the optical center, what happens to the plus lens?

A

This is where they meet.

23
Q

In the optical center, what happens to the minus lens?

A

it is where apex meets apex.

24
Q

What happens when you view a lens through the lensometer?

A

It is the point where the target is exactly centered.

25
Q

What is “against motion”?(only for + lens)

A

While prisms bend light toward the base they also shift the image viewed toward the apex. Because of this, when looking through a moving plus lens you will notice that the image will move in the opposite direction.

26
Q

What is “with motion”?(- lens)

A

when looking through a moving the center of a minus lens, there is no image displacement. You will notice that the image of what you are looking at will move in the same direction.

27
Q

How many dioptric power values do all lenses have?

A

2.
One is the actual power, which may be measured by the lensometer.
The other the effective power that only the wearer perceives.

28
Q

Does the measured power remains the same or changes?

A

It always stays the same but the effective power can change depending on the vertex or fitting distance of the lens from the eye.

29
Q

What happens when plus lenses are moved closer to the eye?

A

their power becomes less effective or weaker.

30
Q

What happens when a plus lens is moved further from the eye?

A

the effect of the power becomes stronger.

31
Q

What happens when a minus lens are moved closer to the eye?

A

their power becomes stronger .

32
Q

What happens when a minus lens are moved further from the eye?

A

their effective power becomes weaker.

33
Q

How do you ensure that the patient is seeing through the intended power?

A

The spectacles should be fit to the same vertex distance used during the refraction.

34
Q

What does opticians use to measure the vertex distance?

A

vertex distometer.

35
Q

What does the distometer measure?

A

The distance from the back of the lens of the eye in mm.

36
Q

How can the effective power be quickly calculated?

A

F=D/1-(dD)
F: effective power
D: true power
d: distance in meters the lens is moved

37
Q

Strong plus or convex lenses that distort a square to a in pincushion shape

A

.

38
Q

Storng minus or ______lenses distort a barrel shape.

A

.

39
Q

What happens when a plus lens is moved further from the eye?

A

the effect of the power becomes stronger.

40
Q

What happens when a minus lens are moved closer to the eye?

A

their power becomes stronger .

41
Q

What happens when a minus lens are moved further from the eye?

A

their effective power becomes weaker.

42
Q

How do you ensure that the patient is seeing through the intended power?

A

The spectacles should be fit to the same vertex distance used during the refraction.

43
Q

What does opticians use to measure the vertex distance?

A

vertex distometer.

44
Q

What does the distometer measure?

A

The distance from the back of the lens of the eye in mm.

45
Q

How can the effective power be quickly calculated?

A

F=D/1-(dD)
F: effective power
D: true power
d: distance in meters the lens is moved

46
Q

Strong plus or convex lenses that distort a square to a in pincushion shape

A

.

47
Q

Storng minus or ______lenses distort a barrel shape.

A

.

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