MNSR 35 - Optical Defects of Vision

Question 1

Refractive power

Answer 1

1/f f= focal length (where rays converge) always measured in metres
if in cm use 0.xm

Question 2

unit used to measure refractive power

Answer 2

(Dioptres, or D)
converge lens = positive value
diverging lens = negative value

Question 3

object distance

Answer 3

distance between object and lens = u

Question 4

image distance

Answer 4

distance between image (in retina) and lens = v

0.025m

Question 5

another way to calculate refractive power

Answer 5

1/u + 1/v

measured in metres

Question 6

when is refractive power at its maximum

Answer 6

when the eye is fully accommodated - fully focussed to see an object at near point

Question 7

when is the refractive power at its minimum

Answer 7

when eye viewed object at infinity

Question 8

power of accommodation

Answer 8

difference between refractive power when it is focussed on an object at near point and when it is focussed on object at infinity

Question 9

Power of accommodation calculation

Answer 9

Maximum refractive power - Minimum refractive power

Question 10

How do we give correct lens to correct defective vision

Answer 10

normal refractive power = defective refractive power + corrective refractive power

Question 11

4 common vision defect

Answer 11

myopia (short sightedness)
hypermetropia (long sightedness)
presbyopia
astigmatism

Question 12

myopia

Answer 12

when the eye is RELAXED the focal point is in front of the retina - can’t see distant objects clearly
short-sightedness
Refractive power is too long (1/f focal length is too short)

Question 13

which lens is used to correct myopia

Answer 13

use diverging lens

Question 14

hypermetropia

Answer 14

in a RELAXED eye the focal length is behind the retina
can’t see close objects
long sightedness
refractive power is too short (Rp = 1/f focal length is too long)

Question 15

how does hypermetropia occur

Answer 15

refractive power is too short or too low

Question 16

how is hypermetropia treated?

Answer 16

using a converging lens

Question 17

presbyopia

Answer 17

the reduction in accommodation that occurs with increasing age
lose focus in close object
aka - old sight
focus length is behind retina

Question 18

how is presbyopia treated?

Answer 18

using converging lens only when required
OR
wear half glasses, bi-focals or vari-focal lenses if they have some other vision defect

Question 19

what should the near point of an eye be

Answer 19

25 cm or 0.25m

Question 20

how to determine what lens of what dioptre should be used?

Answer 20

1. calculate near point needed
2. calculate defective refractive power = 1/u + 1/v
   u = object distance, distance the person can see at v = 0.025
3. calculate the normal refractive power = 1/u + 1/v
   u = distance between lens and object the person SHOULD be able to see
   v = 0.025
4. calculate corrective refractive = Pnormal - Pdefective
5. value is positive = use converging lens
   value is negative = use diverging lens

Question 21

Astigmatism and its causes

Answer 21

an optical defect of the eye where sufferers have distorted vision due to uneven curvature of the cornea

Question 22

How is astigmatism tested

Answer 22

pattern of 3 vertical lines arranged radially of different thickness

Question 23

how is astigmatism corrected?

Answer 23

Astigmatism can be corrected using asymmetric cylindrical lenses whose refractive power is greater in one direction than in the other.

Question 24

what defect in the eyes causes hypermetropia

Answer 24

short eyeball

flat cornea