Test 13  (Optics of low visual aids)

1. The following may be used as low visual aids:

a. text scanner

b. closed circuit television

c. high-add bifocal

d. convex cylinder lens

e. telescope
 

Answera.T   b.T   c.T    d.F   e.T
The following optical devices may be used for low visual aids:
       High-add bifocal
       Prismatic half-frame spectacle
       Head-mounted magnifier
       Hand-held magnifier
       Stand magnifier
       Spectacle-mounted telemicroscope
       Closed-circuit television (CCTV)
       Text scanner, enlarged font on computer monitor

 

2. The disadvantages of using optical magnifying glasses for low visual 
     aids include:

a. reduced depth of  focus

b. reduced visual field

c. reduced contrast

d. objects need to be placed far away

e. unsteadiness of images if the objects are not held steady
 

Answera.T   b.T   c.F   d.F   e.T
Disadvantages of optical magnifying glasses includes: reduced depth of focus, reduced visual fields, objects need to be placed near the eye and unsteadiness of images if the objects are not held steady.
 

3. The following are true about convex lens for low visual aids:

a. the object is placed at its focal point

b. the image produced is virtual

c. the image is erect

d. the field of vision obtained is decreased if the user 
    moved away from the lens

e. the field of vision obtained is reduced if the diameter 
    of the lens is reduced
 

Answera.F   b.T   c.T    d.T   e.T
The object is placed between the focal length and the lens.
The image is magnified and erect. The field of vision
is dependent on the size ie. diameter of the lens and the distance between the eye and the lens (the greater the distance between the lens and the eye, the smaller the visual field).

4. The following are true about the Galilean telescope:

a. it is made up of two convex lenses

b. the distance between the two lenses is the sum of the 
    focal length

c. it is usually lighter than the astronomical telescope

d. it is usually shorter than the astronomical telescope

e. the image produced is usually dimmer than that seen 
    by the naked eye
 

Answera.F   b.F   c.T    d.T   e.T
Galilean telescope is made up of a convex objective and a concave eye-piece lens. The separation between the lenses is the difference between the their focal lengths and for this reason is usually shorter than the astronomical telescope (separation is the sum of the focal lengths) and also lighter due to the shorter 
distance. The image produced is usually dimmer than that seen by the eyes because light is reflected and absorbed as it passes through each lens. 
 

5. The image produced by a Galilean telescope is:

a. laterally inverted

b. upright

c. real

d. at infinity

e. magnified
 

Answera.F   b.T   c.F   d.T   e.T
The image produced by a Galilean telescope is at infinity, virtual, upright and magnified. 

6. The following are true about the Galilean telescope:

a. it is invented by Galileo

b. the objective lens usually has a much lower power than 
    the eye piece

c. the magnification can be calculated by using the the 
    formula M=fo/fe (where fo is the focal length of the
    object and fe the focal length of the eye piece)

d. it is useful for viewing faint stars at night

e. it magnifies by increasing the angle subteneded by 
    the object at the eye
 

Answera.F  b.T  c.T   d.F  e.T
Despite the name it is not invented by Galileo. It is usually made up of a lower power plus objective lens and a high power minus eyepiece lens. The magnification can also be calculated using Fe/Fo where Fe is the dioptre power of the eyepiece and Fo the power of the objective lens. Compare with the astronomical 
telescope, Galilean telescope is not efficient in gathering light and therefore not suitable for viewing distant faint stars. The angle subtended by the object at the eye is increased and hence magnification.

7. The following may be used as visual aid for patients with 
     significant visual field loss due to advanced glaucoma:

a. concave lens

b. inverted Galilean telescope

c. astronomical telescope

d. prism with the base towards the area of scotoma

e. mirror mounted on glasses
 

Answera.T  b.T  c.F  d.T  e.T
There are two optical ways which can help patients with
constricted visual fields: minification and image relocation. Minification of image allows the remaining functional retina to process more information and this can be achieved with concave lens or inverted Galilean telescope. Image relocation relocates visual information 
from the scotoma closer to the area of functioning visual 
field. This has the effect of minimizing the size of the eye or head movement required to detect the visual 
object. This technique typically uses prisms placed nasally and temporally with the base in the direction of the scotoma but can also use mirrors.
Lastly, the non-optical way involves making the patients
aware of the spatial area to be processed ie. behavioural approach and this involves increasing the efficiency of eye movements.

 

 

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