Physiological Optics

Question 1

What are the 2 ocular structures the human eye refracts at

Answer 1

Cornea and lens

-cornea:
fixed focusing
Power approx : + 40D
- 2/3 of total refraction

Lens
-variable focusing powers accommodation
- min power approx + 20 D
- max power approx + 33D- when accommodating
-1/3 of total refraction

TOTAL POWER OF EYE APPROX +60D

Question 2

What are the 3 factors that determine the eyes ability to focus on light

Answer 2

Shape of cornea- power of cornea
Power of the eye
Length of the eye

Question 3

What does a more curved surface mean

Answer 3

The more powerful it is so the more it refracts light

Question 4

When does emmetropia occur

Answer 4

-Occurs when there is matching between the power of the eyes optical system (lens and cornea) and the length of the eye
- light focused on the retina- the eye is emmetropic
- results in CLEAR VISION

Question 5

Emmetropia- match

Answer 5

• objects at infinity- meaning light is leaving the object parallel
• focus at the retina
• no accommodative effort
  -60D
  -40D CORNEA
  -20D LENS

Question 6

What is refractive error

Answer 6

-It occurs when there is a mismatch between the power of the eyes optical system (dioptric power of lens and cornea) and the length of the eye.
-Light is not focus on the retina -the eye is ametropic
- generally results In Blurry vision

Question 7

Ametropia- mismatch

Answer 7

• light not focusing at the retina
  -No accommodative effort
• blue circle projected

Question 8

What are 3 basic types of refractive error

Answer 8

-myopia
- hyperopia
- astigmatism

Question 9

Myopia

Answer 9

• focused in front of the retina
  -blue circle projected on retina
• objects nearby are clear but far distance are blurry
• no accommodative effort

Question 10

When does myopia occur?

Answer 10

Occurs when:
-cornea is too curved
- Lena is too powerful
- eye is too long
- or a combination of these

Question 11

Hyperopia

Answer 11

• object at infinity
• no accommodative effort
  -light focused behind retina
  -blue circle

Question 12

When does hyperopia occur

Answer 12

-cornea is to flat
-lens is to weak
-eye is to short
- or a combination of these things

Question 13

How does hyperopia look

Answer 13

Far distance is clear and up close is blurry

Question 14

Astigmatism

Answer 14

-refractive error of the eye produces two foci
- an elliptical blur (2 blur circles overlapping) is imaged on the retina

Question 15

When does astigmatism occur

Answer 15

-cornea and/or the lens have different curvatures in different meridians

Question 16

What are the types of astigmatism

Answer 16

Regular astigmatism
-axis with greatest curvature and axis with least curvature are at 90 angle to one another

Irregular astigmatism
-irregularities in the curvature conform to No particular geometry

Question 17

Accommodation

Answer 17

-Change in the refractive power of the eye by a change in shape of the crystalline lens
-associated with convergence

Question 18

Presbyopia (loss of accommodative ability

Answer 18

-reduction/loss of ability to accommodate with age
-crystalline lens loses its ability to change shape
-decreased ability to focusing on close things
-Arms are not long enough
-still an impulse to focusing hence converge