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Test 20

1. The following are true about astronomical telescope: a. the image is real b. the image is inverted c. the image is magnified d. the distance between the objective lens and the eyepiece      is the sum of the focal lengths. e. most binoculars are made up of astronomical telescope   Answer a. T   b.T   c.T   d.T   e.T 2. The following are true about Galilean telescope: a. the objective lens is a concave lens b. the distance between the objective lens and the eyepiece      is equal to the focal length of the objective lens c. it is the preferred form of telescope for patients with low      vision d. prism is needed to give the viewer and erect image e. it can be created by holding a +3.00 D lens 25 cm from      an aphakic eye    Answer a.F   b.T   c.T   d.F   e.T The objective lens is a convex lens. It is the preferred form of telescope because the distance is shorter and prism is not needed to invert the image as is the case in astronomical telescope. 3. In patients with low vision, telescope used for distant viewing can      be modified for close viewing by: a. increasing the length of the telescope tube b. increasing the focal length of the objective lens c. increasing the diameter of the eyepiece d. adding plus lens behind the ocular of the telescope e. adding plus lenses in front of the objective lens Answer a.T   b.F   c.F   d.T   e.T a, d and e are three ways in which the telescopes can be modified  for close viewing. 4. The disadvantages of telescope as a device for low visual aid include: a. reduced brightness b. ring scotoma c. reduced field of vision d. parallax e. reduced depth of focus   Answer a.T   b.T   c.T   d.T   e.T   5. In patients with severe visual field loss, the following devices can      be used to enhance the visual fields: a. Fresnel prism with the base in the direction of the field      defect b. reverse telescope c. convex lenses d. convex mirrors e. increased contrast   Answer a.T   b.T   c.F   d.T   e.F Optical devices that minify an object can be used  to increase the field of vision and these include reverse telescopes, concave lenses and convex mirrors. Fresnel prism works by shifting the image.   Return to the main page