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DISPENSING > prescription and lenses ( lecture 2) > Flashcards

prescription and lenses ( lecture 2) Flashcards

1
Q

What does a lens do?

A

can describe the action of a lens by looking at its focal length and principle focus

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2
Q

What is a principle focus?

A

point at which it focuses light coming from infinity

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3
Q

What is the 1st principle focus?

A

when light is travelling from right to left.

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4
Q

What is the 2nd principle focus (positive lens)?

A

(WE ASSUME LIGHT TRAVELS FROM LEFT TO RIGHT) the light converges to a real image at its 2nd principal focus (F’)
(light from infinity represents straight parallel lines (left) and passes through lens and light converges at a point (F’) )

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5
Q

What is F’?

A

2nd principle focus

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6
Q

What is f’?

A

2nd principle focal length- describes distance from lens to its principle focus (f’)

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7
Q

WHAT IS a focal length?

A

focal length of a lens is from the vertex of the lens to th 2nd principle focal point

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8
Q

What is a negative lens?

A

diverges light

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9
Q

What is the 2nd principle focus (negative lens)?

A

its always negative

-to find focal point need to find virtual point from where the light diverges from.

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10
Q

What is an optical axis?

A

where light passes through lens and deviated.

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11
Q

What does it mean when you have a lens with short focal length?

A

it is more powerful, stronger, steeper curved lens

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12
Q

What does it mean when you have a lens with a longer focal length?

A

it is a less powerful, weaker , a flatter curved lens

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13
Q

What is the power of lens measured in?

A

D (Dioptres)

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14
Q

What is the power of a lens in dioptres?

A

the reciprocal of its focal length in metres (m) D=1/f’

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15
Q

How do we measure Dioptres?

A

0.25D steps

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16
Q

What do dioptres represent?

A

the power of spheres or cylinders

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17
Q

How must D be written?

A

with a + or - sign and MUST be to 2 DECIMAL PLACES

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18
Q

What is emmetropia?

A

light from point objects in all distances are focused on retina as a point image - which means image is clear
-No optical correction required to patient

19
Q

What is the axial length of eye?

A

24mm

20
Q

Overall power of optical system?

A

58D

21
Q

What is hypermetropia (long sightedness)?

A

-condition when the incident light is focused behind the retina - blurred image

22
Q

What are the 2 reasons of hypermetropia?

A
  • optical power of system is too weak to focus light

- overall dimensions of light are too short

23
Q

How do we correct hypermetropia?

A

correct with a positive spherical lens

24
Q

What is spherical lens?

A

has the same power in every direction

25
Q

What is the principle of this correction?

A

to make sure the 2nd focal point of lens co insides with the far point of the eye

26
Q

What does the positive spherical lens do for

A

increases convergence and brings the light forward onto the retina (image on retina) as focal point is brought forward.

27
Q

What is accomodation?

A
  • some patients have ability to increase optical power of the lens by making it thicker and convex
  • ability to do this reduces from age
28
Q

What is the effect of accomodation?

A

can cause eye strain (astenopia)

29
Q

What is myopia?

A

incident light focused infront of retina leaving a blurred image on retina

30
Q

How do we correct myopia ?

A

with a negative lens

31
Q

What is negative spherical lens?

A

diverges incident light before it reaches cornea and patient sees a virtual image
- When diverging a negative lens placed infront of the cornea and light is focused on the retina and can see clear image

32
Q

What does astigmastism mean?

A

non-point forming

33
Q

a general patients eye being able to see clearly?

A

when cornea is spherical, which means all rays entering the eye focus on a single point on the retina

34
Q

What is astigmatism?

A
  • when the patients cornea is aspherical therefore power of lens is different in different directions.
  • it causes light to be bent more in one direction more than other (so a higher power than the other)
  • this creates 2 principal powers , a maximum power in one meridian and minmum power in the 2nd meridian jwhich falls perpendicular (90) to the first.
  • This means a patient looking at a point object doesnt become a point object on the retina- which means the image is distorted and blurred on the retina.
35
Q

How is astigmatism formed on a diagram ?

A

2 line foci produced instead of a one dot image

36
Q

What does astigmatism result in?

A

results in a image blurry more in one direction than the other

37
Q

How to correct astigmatism?

A

use cylindercal lens

  • Lens must have 2 principal powers at perpendicular merdians to one another
  • Simplest form of achieveing 2 perpendicular powered meridians is on the basis of a cylinder
  • these lenses have 1 surface with no power which is 90degrees away from a surface with maximum power.
38
Q

What is an axis meridian?

A

surface of cylinder with no power (flat side)

39
Q

What is the power meridian?

A

surface with maximum power ( curved)

40
Q

What is the power meridian?

A

surface with maximum power ( curved)

41
Q

What is the cylinder power?

A

90 degrees from axis

42
Q

What are toric lenses?

A

sum of spherical and cylindercal lenses for patients with hypermetropia, myopia and astigmatism.

43
Q

how to write these prescriptions?

A

1) how myopic or hypermetropic patient is (+1.00/or/-1.00)
2) 2nd component represents power of cylinder (as astigmatic so been corrected)
3) axis and power of cylinder lies in 3rd component
+1.00/-0.75 x180

44
Q

How to write prescription in positive cyl?

A

+1.00/-0.75 x180

```
Transpose prescription
+0.25/+0.75 x90
add spherical and cylindercal
(keep cylinder positive)
(-90degrees)
~~~

DISPENSING (19 decks)

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