An old man cannot see objects closer than 1 m from the eye clearly. Name the defect of vision he is suffering from. How can it be corrected ? Draw ray diagram for the (i) defect of vision and also (ii) for its correction.
An old man cannot see objects closer than 1 m from the eye clearly. Name the defect of vision he is suffering from. How can it be corrected ? Draw ray diagram for the (i) defect of vision and also (ii) for its correction.
Official answer
1. Name of the Defect
The old man is suffering from Hypermetropia (far-sightedness) or Presbyopia.
- Hypermetropia is a defect of vision where a person can see distant objects clearly but cannot see nearby objects distinctly. Here, the near point of the eye has receded from the normal to .
- Since the person is old, this defect is specifically referred to as Presbyopia, which arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens with age.
2. Correction
This defect can be corrected by using a convex lens of appropriate power. The converging lens provides the additional focusing power required to bring the light rays from a nearby object back onto the retina.
3. Ray Diagrams
``diagram {"xrange": [27.8, 567.2], "yrange": [-328.0, 13.0], "lines": [{"from": [150.0, -100.0], "to": [412.0, -90.0], "label": ""}, {"from": [150.0, -100.0], "to": [412.0, -110.0], "label": ""}, {"from": [412.0, -90.0], "to": [505.0, -100.0], "label": ""}, {"from": [412.0, -110.0], "to": [505.0, -100.0], "label": ""}, {"from": [150.0, -280.0], "to": [320.0, -265.0], "label": ""}, {"from": [150.0, -280.0], "to": [320.0, -295.0], "label": ""}, {"from": [320.0, -265.0], "to": [412.0, -270.0], "label": ""}, {"from": [320.0, -295.0], "to": [412.0, -290.0], "label": ""}, {"from": [320.0, -265.0], "to": [80.0, -280.0], "label": ""}, {"from": [320.0, -295.0], "to": [80.0, -280.0], "label": ""}, {"from": [412.0, -270.0], "to": [485.0, -280.0], "label": ""}, {"from": [412.0, -290.0], "to": [485.0, -280.0], "label": ""}], "points": [{"x": 300.0, "y": -20.0, "label": ""}, {"x": 450.0, "y": -100.0, "label": ""}, {"x": 150.0, "y": -100.0, "label": "N' (Normal Near Point, 25 cm)"}, {"x": 150.0, "y": -115.0, "label": ""}, {"x": 505.0, "y": -100.0, "label": ""}, {"x": 515.0, "y": -104.0, "label": ""}, {"x": 300.0, "y": -200.0, "label": ""}, {"x": 450.0, "y": -280.0, "label": ""}, {"x": 150.0, "y": -280.0, "label": ""}, {"x": 150.0, "y": -295.0, "label": ""}, {"x": 80.0, "y": -280.0, "label": ""}, {"x": 80.0, "y": -295.0, "label": ""}, {"x": 485.0, "y": -275.0, "label": ""}], "caption": "Hypermetropic Eye and its Correction"} ``
Marking scheme
- 1.Identification of the defect of vision (Hypermetropia / Presbyopia) (1 mark)
- 2.Stating the correct method of correction (using a convex lens of appropriate power) (1 mark)
- 3.Drawing neat, labeled ray diagrams for both the defect and its correction (1 mark)
Hint
Think about which type of lens is converging in nature to help an eye whose focal length has become too long.
Quick oral answer
The defect is Hypermetropia or Presbyopia. It is corrected using a convex lens because a convex lens converges the light rays before they enter the eye, helping the weakened eye lens focus the image precisely on the retina instead of behind it.
Common mistakes
- 1.Confusing hypermetropia with myopia and suggesting a concave lens.
- 2.Drawing the light rays focusing in front of the retina instead of behind the retina for the defective eye.
- 3.Forgetting to draw arrows on the light rays to indicate their direction.
Analysis and explanation
The question describes a scenario where an old man cannot see nearby objects clearly (his near point has receded to instead of the normal ). This is a classic symptom of Hypermetropia (far-sightedness). Since the individual is elderly, this is specifically caused by Presbyopia, which is the age-related loss of accommodation power due to the weakening of ciliary muscles and hardening of the crystalline lens.
To correct this, a convex lens is placed in front of the eye. The convex lens converges the diverging rays coming from the normal near point () so that they appear to diverge from the virtual near point () of the defective eye. The eye lens then easily focuses these rays directly onto the retina, restoring clear near vision.
Interesting facts
The normal near point of a young adult is about , but as we age, the ciliary muscles weaken, causing this near point to recede significantly, which is why many older adults require reading glasses.
Question details
Q22 of 41 questions
- Board
- CBSE
- Class
- Class 10
- Subject
- Science
- Year
- 2012
- Section
- Section A
- Marks
- 3
- Type
- Short Answer
- Topic
- Defects of Vision and Their Correction
- Bloom's level
- Applying
- Syllabus topic
- Refractive defects of vision and their correction (Myopia, Hypermetropia, Presbyopia)
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Frequently asked questions
How many marks does this question carry in CBSE Class 10 Science 2012?
This question carries 3 marks in the CBSE Class 10 Science 2012 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 2012 paper?
This is a Short Answer question from Section A in the CBSE Class 10 Science 2012 paper.