Test 7

. 1. True statements about slit-lamp include: a. contains two convex lenses separated by a distance greater than the sum of their focal      lengths b. the image formed is real, erect and magnified c. Porro prism is found in the slit-lamp to invert the image formed d. Porro prism in the slit-lamp reduces the length of the microscope e. variable magnification can be achieved by using zoom lenses. answera.T   b.F   c.T   d.T  e.T Most slit-lamp used nowadays is a binocular compound microscope. Therefore, the two convex lens (ie. objective and eyepiece lenses) are separated by a distance greater than the sum of their focal lengths. The image produced is virtual, erect and magnified. 2. The following are true about the direct ophthalmoscope: a. the retina image of a hypermetropic patient appears smaller than that of a myopic patient b. a divergent beam of light is used to illuminate the patient's retina c. a -2.00D observer (without glasses) who needs +2.00D lens to examine a patient      (without glasses on) suggests that the later is hypermetropic. d. up to X15 magnification can be achieved  e. it is a sensitive tool for detecting macular oedema answera.T   b.T   c.T   d.T   e.F In ametropia, the lenses of the patients and the direct ophthalmoscope act as a Galilean telescope. Therefore, a myopic fundus is magnified whereas the hypermetropic one reduced. The sum of the observer's and the patient's ametropia give the power of the lens needed to examine the patient's fundus. Macular oedema especially if subtle can be difficult to detect. Therefore, an indirect ophthalmoscope which give a stereoscopic view is preferred. 3.In slit-lamp examination: a. specular reflection is useful in examining the corneal endothelium b. the angle of incidence and reflection are equal in specular reflection c. total internal reflection is used in sclerotic scatter d. in retroillumination, a secondary light source is used to illuminate a more anterior surface e. corneal oedema is best highlighted with retroillumination answera.T   b.T   c.T   d.T   e.F Corneal oedema and opacities are best highlighted with sclerotic illumination. 4. With regard to von Helmholtz keratometer: a. it uses the first Purkinje-Sanson image b. the image is real, erect and magnified c. it uses a circular object of known size and distance  d. the image size varies with corneal curvature.  e. Wollaston prism is used to produce image doubling. answera.T   b.T   c.T   d.T   e.F Rotating glass plates are used in von Helmholtz keratometer to produce image doubling. 5. In indirect ophthalmoscope: a. the illumination and viewing axes are co-axial. b. as the condensing lens is moved away from the patient, the image size is determined by      the refractive status of the patient. c. the higher the power of the condensing lens, the smaller the image size d. the field of illumination is wider in myopes than hypermetropes e. the field of vision is wider with a +20D condensing lens than +30D lens. answera.F   b.T   c.T   d.T   e.F The illumination and viewing axes are non-coaxial. The size of the image is affected by: the refractive status of the patient and the distance from which the observer see the image: in myopes, the image increases as the condensing lens is moved away from the patient; the converse is true for hypermetropes. the power of the condensing lens; a lower power give a higher magnification. The field of view is determined by the observer's pupil size, image magnification and the diameter of the condensing lens.  The field of illumination is determined by the patient's pupil size and refractive status.

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