Chapter 13: Optics of Low Vision Aids

Question 1

Define the magnifying power of an optical system (MP)

Answer 1

Retinal image with instrument/ retinal image without instrument

Question 2

How does a convex magnifying lens work

Answer 2

Similar to a loop, allows the eye to view objects at close range and increases the angle subtended by the eye.

Question 3

Using a concave magnifying glass, where in the ray diagram is the object placed?

Answer 3

Between f1 and the lens to produce a virtual, erect and enlarged image.

Question 4

Using a convex magnifying glass, what happens to the image when the object moves closes the F1

Answer 4

It gets larger and so substends bigger angle at the eye.

Question 5

In a magnifying convex lens, what happens to the field of view if the eye lens distance is increased?

Answer 5

Field of view decreases (angle subtended at the eye increases and magnification increases)
Need to move lens around move to see peripheries of image.

Question 6

In a Galilean telescope, the convex and concave lenses in the system are separated by what?

Answer 6

Difference in their focal lengths.

Question 7

How do Galilean telescopes work and what kind of image does it create?

Answer 7

Increase the angle substended at the eye.
Creates a magnified, virtual, erect image

Question 8

How do you calculate the magnification of a Galilean telescope

Answer 8

Power of eyepiece lens/ power of objective lens

Can also calculate angular magnification using angle of emergency/ angle of incidence

Question 9

What are the limitations of a Galilean telescope?

Answer 9

1. increased magnification means reduced field of view.
   2.the object needs to be held very close to the eye.
   3.increased magnification reduces depth.

As a result, movement of hand causes unstable image. So prescriber should give lowest magnification that fits the patients requirements