1. What is visual acuity 2. Record your results from the Snellen eye chart here
ID: 142814 • Letter: 1
Question
1. What is visual acuity 2. Record your results from the Snellen eye chart here Right eye Left eye There should be two numbers for each eye. What does the first number represent? What does the second number represent? 3. Explain the cause of astigmatism. 4. Define accommodation. 5. What was your near point? Rt eye Left eye 6. When observing the pupillary response, when the right eye received light, what did the left eye do? 7. What is the blind spot? When using the opthalmoscope, describe what you saw 8. After staring at the red square, describe what you saw on the white sheet. Explain this phenomenon.Explanation / Answer
1. Clarity of vision or how clearly you can see is referred as visual acuity. It depends on optical and neural factors like a) the sensitivity of the interpretative faculty of the brain b) the health and functioning of the retina, and c) the sharpness of the retinal focus within the eye, 20/20 vision means you can see anything from 20feet distance is expressed as normal visual acuity.
2. Right eye--- 6/ 12
Left eye---- 6/12
The first number represents the distance at which the chart is kept away from the person. Here 6 means the chart is kept at 6 metres or 20 feet away from the person to read the smallest line in the chart.
Second letter represents the distance to the smallest line a normal person is able to read, in same chart. Here 12 metres is the distance for a person with normal visual acuity, to read the smallest line letters in the same chart
Thus 6/12 visual acuity represents person needs to approach to a distance of 6 metres (20 ft) to read letters that a person with normal acuity could read at 12 metres (39 ft).
3. Astigmatism is a refractive error caused by an error in the shape of the cornea. The front surface of the eye like lens or cornea has an irregular curve which makes changes in the way light passes or refracts to the retina. This can lead to blurry, fuzzy or distorted vision.
4. The focusing ability of the eye from distant to near objects and near to distant objects is called accommodation of the eye. This is by adjusting the focal length of a lens by which the eye focuses the object on the retina as its distance from the eye varies.
5. Near point is the nearest point of the eye at which the placed object can be seen clearly with sharp image on the retina. Usually the nearest point is 25cm from the eye. Right eye : 24.5cm Left eye: 25 cm
6. When observing the pupillary response the right eye received light. The pupil of the right eye constricts. This is to avoid the amount of damaging light passing through the eye. The left eye also constricts in the same manner simultaneously. In healthy eyes, the bright light shone onto one eye constcits both pupils equally. This is consensual light reflex. This is because, the reaction of the pupils on the right eye and the left eye are linked. Part of the optic nerve from each eye crosses over each other and couples to the muscles controlling the size of the pupil. Thus the bright light shone onto one eye can equally affect or constrict the pupil of the other eye too.
7. . The inner part of the eye where the light absorbing photoreceptor cells- rods and cones- are not present on the retina, is called a blond spot or optic disc. The objects of which the images falling on this part of retina are not visible to the person. When look through an ophthalmoscope, the parts of the retina are seen. a) fovea is the dark area and a depression in the retina b) Macula lutea is a yellow pigmented area in foveal area. c) blind spot or optic disc where all the blood vessels and optic nerves converge and go out of the retina to the brain.
8. Cone cells are stimulated by light rays and receive them to produce an image on the retina. When a person look at a red object, red cone cells are stimulated and the object is viewed. On staring a red square for long, the red cone cells get fatigued and temporarily do not respond. When you look on the white sheet, all the other cone cells s rather than the red are working and the complementary colour appears. This lasts until the fatigued cone cells are recovered and functional. This phenomenon is called afterimage.
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