A person may suffer from both myopia and hypermetropia defects.
(a) What is this condition called?
(b) When does it happen?
(c) Name the type of lens often required by the persons suffering from this defect. Draw labelled diagram of such lenses.
A person may suffer from both myopia and hypermetropia defects.
(a) What is this condition called?
(b) When does it happen?
(c) Name the type of lens often required by the persons suffering from this defect. Draw labelled diagram of such lenses.
Official answer
(a) Name of the condition: This condition is called Presbyopia (specifically when a person suffers from both myopia and hypermetropia simultaneously).
(b) When it happens: It usually occurs with ageing. It arises due to the gradual weakening of the ciliary muscles and the diminishing flexibility of the crystalline eye lens.
(c) Type of lens required: Such people often require bi-focal lenses to correct their vision.
- The upper portion of a bi-focal lens consists of a concave lens to facilitate distant vision.
- The lower portion consists of a convex lens to facilitate near vision.
``diagram {"xrange": [149.0, 151.0], "yrange": [-142.2, -67.8], "points": [{"x": 150.0, "y": -100.0, "label": ""}, {"x": 150.0, "y": -75.0, "label": ""}, {"x": 150.0, "y": -135.0, "label": ""}], "caption": "Bi-focal Lens Diagram"} ``
Marking scheme
- 1.State the name of the condition as Presbyopia. (1 mark)
- 2.Explain that it happens with ageing due to weakening of ciliary muscles and loss of flexibility of the eye lens. (1 mark)
- 3.Identify bi-focal lenses (with concave upper and convex lower parts) and draw a clearly labelled diagram. (1 mark)
Hint
Think about the age-related defect where both near and far vision are impaired, requiring a single spectacle lens with two distinct optical zones.
Quick oral answer
Presbyopia is corrected using bi-focal lenses. The upper part is a concave lens for distant vision, and the lower part is a convex lens for near vision.
Common mistakes
- 1.Confusing presbyopia with simple hypermetropia.
- 2.Swapping the positions of the concave and convex lenses in the bi-focal lens diagram (remember: we look straight/up for distance, and look down to read).
Analysis and explanation
Presbyopia is an age-related refractive defect. As a person grows older, the power of accommodation of the eye decreases because the ciliary muscles lose their strength to contract/relax effectively, and the crystalline lens loses its elastic flexibility. When a person suffers from both myopia (inability to see distant objects clearly) and hypermetropia (inability to see nearby objects clearly), a single-focal lens is insufficient. Hence, a bi-focal lens is designed. The upper half acts as a concave lens to diverge distant light rays, while the lower half acts as a convex lens to converge light rays coming from nearby reading materials.
Interesting facts
Today, instead of traditional bi-focal lenses with a visible dividing line, progressive lenses (or contact lenses) are widely used to provide a seamless transition between distant and near focal points.
Question details
Q23 of 30 questions
- Board
- CBSE
- Class
- Class 10
- Subject
- Science
- Year
- 2020
- Section
- Section B
- Marks
- 3
- Type
- Short Answer
- Topic
- Defects of Vision and Their Correction
- Bloom's level
- Understanding
- Syllabus topic
- Defects of vision and their correction
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Frequently asked questions
How many marks does this question carry in CBSE Class 10 Science 2020?
This question carries 3 marks in the CBSE Class 10 Science 2020 examination.
What topic does this question cover in Science?
This question covers the topic "Defects of Vision and Their Correction" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2020 paper?
This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.