mnsr 34

Question 1

the refractive power is given by

Answer 1

P = 1/f (Dioptres, or D)
focal length needs to be in metres to use D
( from cm to metre x0.01)

Question 2

for the calculation of corrective lens for defective vision

Answer 2

• image( v) distance will ALWAYS be the length of the eyeball ~ 2.5 cm ( aka the distance from the front of the cornea to the retina )

Question 3

The refractive power of the eye is maximum when

Answer 3

the eye is fully accommodated and fully focused in the near point

Question 4

the refractive power is minimum when

Answer 4

its in infinity ( u ) and the v will be 2.5 cm , relaxed

Question 5

p normal =

Answer 5

p defective ( the actual power of the defective eye) + p corrective ( power required to correct )
p normal ( what we’re trying to achieve )

Question 6

The 4 common defects of the eyes

Answer 6

Myopia (short-sightedness).

Hypermetropia (long-sightedness). Also called Hyperopia.

Presbyopia (old-sight).

Astigmatism.

Question 7

myopia is when

Answer 7

-parallel light from a distant object is focussed by the relaxed eyeIN FRONT of the retina, i.e.
- can’t focus on an object further away from the far point
-the farthest distance over which a person can see objects clearly with a relaxed eye (infinity for a normal person).
- lens used: divergent concave lens placed infront of the eye allowing the image to be focused on the retina

Question 8

A myopic person has a far point at a distance of 1 m for both eyes. What corrective lens would you recommend for this person?

Answer 8

1- P defective = 1/u + 1/v = 1/(1) + 1/(0.025) = 41 Dioptres
2- P normal = 1/u + 1/v = 1/infinity + 1/(0.025) = 40 Dioptres
3- Pcorrective = Pnormal – Pdefective
= 40 D - 41 D
= -1 D

Question 9

—- Parallel light rays entering the relaxed eye are brought to a focus BEHIND the retina

Answer 9

hypermetropia

Question 10

causes of the hyperopia

Answer 10

• refractive power of the eye is too low
• eyeball too short
• requires convergent convex lens
• problem w/ the near point

Question 11

causes of myopia

Answer 11

• eyeball too big
• problem w the far point, its limited instead of infinity

Question 12

presbyopia refers to the

Answer 12

old sight , reduction of accommodation due to increasing in age
- near point becomes too further away
- distant vison is not affected so no need for a lens

Question 13

people w/ presbyopia can use:

Answer 13

• corrective convergent lens as: reading glasses only when required
• wear half glasses, bi-focals or various focal lenses

Question 14

An elderly person finds that they have to hold a book at a distance of 0.7 m from their eyes to read it comfortably. What corrective lenses would you recommend?

Answer 14

• the near point is 0.7m instead of 25cm
  1- P defective = 1/u + 1/v = 1/(0.7) + 1/(0.025) = 1.43 + 40 = 41.43 Dioptres
  2-P normal = 1/u + 1/v = 1/(0.25) + 1/(0.025) = 4 + 40 = 44 Dioptres
  3- => Pcorrective = Pnormal – Pdefective
  = 44 D - 41.43 D
  = 2.57 Dioptres or focal length: 39 cm

Question 15

asigamstism is when

Answer 15

• optical defect of the eye where sufferers have distorted vision due to uneven curvature of the cornea (i.e. the refractive power of the eye is different in the horizontal and vertical directions).

Question 16

Astigmatism can be corrected using

Answer 16

asymmetric cylindrical lenses whose refractive power is greater in one direction than in the other