Practical: Physiology of vision

Question 1

What is meant by emmetropia?

Answer 1

Normal vision - normal refraction of light by the lens so image converges on retina

Question 2

What is meant by hyperopia?

Answer 2

Long sighted-ness - too little refractive power of lens so image converges behind retina

Question 3

What is meant by myopia?

Answer 3

Short sightedness - too high refractive power of lens so images converges before retina

Question 4

How can hyperopia be correct?

Answer 4

Using a convex lens which refracts the light slightly before it enters the eye

Question 5

How can myopia be corrected?

Answer 5

Using a concave lens which causes light rays to diverge slightly before entering the eye

Question 6

In Snellen’s test, how far should the subject stand from the test sheet?

Answer 6

6m

Question 7

How would a normal person read the snellen test?

Answer 7

Should be able to read the biggest line from 60m and the smallest line from 6m

Question 8

How are the findings of Snellen’s test presented?

Answer 8

Given as a ratio of d/D
d = distance from type (6m)
D = distance at which a person with normal vision should be able to read the smallest line the subject can (this is the number given above the line)

Question 9

What would a result of 6/6 in the Snellen test tell you?

Answer 9

You had normal vision

Question 10

Why is the duochrome test used in testing vision?

Answer 10

Its an alternative test used for the purposes of refraction which can take the place of or supplement Snellen’s test

Question 11

How does the duochrome test work?

Answer 11

2 sets of tests provided - one illuminated by red and the other by green light
Green light is refracted more than red light
Someone with myopia will thus be able to see the red better than the green (as there lens has too high refractive power, the red light is refracted less and thus the image more likely to fall on the retina)
Someone with hyperopia will see the green better than the red (there lens doesnt refract enough, if green is refracted more the image is thus more likely to fall on the retina)

Question 12

Why do you a smaller field of colour vision than black and white vision?

Answer 12

Rods = black and white vision and are found more at periphery of retina
Cones = colour vision and found more in center of retina