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Topic Content:
- Long Sightedness (Hypermetropia)
- Short Sightedness (Myopia)
- Astigmatism
- Presbyopia
When the eye is not functioning properly, they are said to suffer defects of vision. The two defects common are Long Sight and Short Sight.
Long Sightedness (Hypermetropia):
A person suffering from long-sightedness can see far objects clearly but cannot see near objects. Sharp images of near objects are formed behind the retina, hence the near point of a long-sighted person is more than 25 cm.
Correction: This can be corrected using a convex lens.
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Example 8.5.1:
Bola cannot read when he holds books closer than 100 cm.
(a) Name the defect
(b) Describe a lens of suitable power
Solution
(a) Things closer than 100 cm will appear blurry to Bola, which is abnormal compared to a person with normal vision. A person with normal vision cannot see images closer than 25 cm clearly but can definitely see things between 100 cm and 25 cm clearly. The defect Bola has is called long sightedness or hypermetropia .
(b) P = \(\frac{1}{f} \)
- P = Power of lens = measured in dioptres (D)
- f = focal length (Note: it is measured in
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