Test 16

1. The following are true about the 3 and 9 O'clock staining      of the cornea: a. they represent areas of poor wetting b. they occur in incomplete blink response c. occur more commonly with gas permeable contact lens      than soft contact lens d. a lens with greater movement may solve the problem e. give rise to corneal vascularization if untreated   answera.T   b.T   c.T   d.T   e.T This problem is due to incomplete wetting of the cornea.  It is commonly seen in gas permeable lenses  which has a smaller diameter than the cornea  and thus leave this area exposed to drying.     2. The following are true: a. the cornea has more refractive power than the lens b. the centre of the cornea is steeper than the periphery in     normal patient c. radial keratotomy flattens the cornea d. hypermetropia can be corrected by steepening the cornea e. strabismus surgery can induce astigmatism. answera.T   b.T   c.T   d.T   e.Ts   3. A Galilean telescope used in low vision: a. has a plus objective b. contains a minus eyepiece lens. c. the distance between the plus objective and the eyepiece is      equal to the sum of the focal lengths of the two lenses d. produces a virtual image e. required an image-inverting system. answera.T   b.T   c.T   d.T   e.F The image produced is virtual, magnified and erect. Therefore, an inverting system is not needed.     4. The following are true about blurred image resulted from tilted retina: a. the blurred image is the result of astigmatism. b. it can be corrected with spectacle. c. it can be corrected with contact lenses. d. refractive surgery can correct the blurred vision. e. cataract extraction with implant can sharpen the image answera.F  b.F   c.F  d.F   e.F A tilted retina can not cause astigmatism, the blurred  vision is a result of curvature of field. It can not be  corrected by modifying the optical medium.   5. The following are true about the contact lens: a. Dk values measured the oxygen permeability of a contact lens b. the oxygen permeability depends on the thickness of the lens c. rigid gas permeable lens always have a lower Dk than soft     contact lens d. PMMA contact lens do not allow oxygen penetration e. the higher the water content of a soft contact lens the higher      the oxygen transmissibility. answera.T   b.T   c.F   d.T   e.T The Dk value is the diffusion coefficient of oxygen and depends on the material of the lens (D) and the solubility  constant of oxygen in the material concerned (k). New rigid gas permeable contact lenses made from fluoropolymer and silicon acrylate have Dk higher than soft contact lenses.   Return to the main page