A student is unable to see clearly the words written on the blackboard placed at a distance of approximately 4 m from him. Name the defect of vision the boy is suffering from. Explain the method of correcting this defect. Draw ray diagram for the: (i) defect of vision and also (ii) for its correction.
A student is unable to see clearly the words written on the blackboard placed at a distance of approximately 4 m from him. Name the defect of vision the boy is suffering from. Explain the method of correcting this defect. Draw ray diagram for the: (i) defect of vision and also (ii) for its correction.
Official answer
1. Name of the Defect
The student is suffering from Myopia (also known as near-sightedness). A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. Since the blackboard is at a distance of (which is beyond the normal near point and represents a distant object for a myopic eye whose far point has receded), the image is formed in front of the retina.
2. Method of Correction
This defect can be corrected by using a concave lens of suitable power. The concave lens diverges the parallel light rays coming from the distant object so that they appear to originate from the far point () of the myopic eye. The eye lens then focuses these rays precisely on the retina, restoring clear vision.
3. Ray Diagrams
``diagram {"xrange": [-19.4, 284.4], "yrange": [-305.6, 10.6], "lines": [{"from": [20.0, -90.0], "to": [130.0, -90.0], "label": ""}, {"from": [20.0, -110.0], "to": [130.0, -110.0], "label": ""}, {"from": [130.0, -90.0], "to": [190.0, -100.0], "label": ""}, {"from": [130.0, -110.0], "to": [190.0, -100.0], "label": ""}, {"from": [10.0, -250.0], "to": [70.0, -250.0], "label": ""}, {"from": [10.0, -270.0], "to": [70.0, -270.0], "label": ""}, {"from": [70.0, -250.0], "to": [130.0, -245.0], "label": ""}, {"from": [70.0, -270.0], "to": [130.0, -275.0], "label": ""}, {"from": [130.0, -245.0], "to": [235.0, -260.0], "label": ""}, {"from": [130.0, -275.0], "to": [235.0, -260.0], "label": ""}], "points": [{"x": 10.0, "y": -20.0, "label": ""}, {"x": 130.0, "y": -100.0, "label": "Eye Lens"}, {"x": 190.0, "y": -100.0, "label": "Image"}, {"x": 255.0, "y": -105.0, "label": ""}, {"x": 10.0, "y": -180.0, "label": ""}, {"x": 130.0, "y": -260.0, "label": ""}, {"x": 70.0, "y": -260.0, "label": "Concave Lens"}, {"x": 235.0, "y": -260.0, "label": "Focus on Retina"}], "caption": "Myopic Eye and its Correction"} ``
Marking scheme
- 1.Correctly identifying the defect as Myopia. (1 mark)
- 2.Stating that a concave lens of suitable power is used for correction. (1 mark)
- 3.Explaining how the concave lens works (diverging rays to focus on the retina). (1 mark)
- 4.Drawing a neat, labelled ray diagram of the myopic eye showing focus in front of the retina. (1 mark)
- 5.Drawing a neat, labelled ray diagram of the corrected myopic eye using a concave lens. (1 mark)
Hint
Since the student cannot see distant objects (like a blackboard at ) clearly but can read nearby books, identify the defect that affects distant vision and recall which type of lens diverges light rays.
Quick oral answer
The student is suffering from myopia. It is corrected using a concave lens because it diverges the incoming parallel rays so they focus directly on the retina.
Common mistakes
- 1.Identifying the defect as hypermetropia.
- 2.Drawing a convex lens instead of a concave lens for correction.
- 3.Showing the corrected image still forming in front of or behind the retina instead of exactly on it.
Analysis and explanation
Myopia occurs due to either (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball. Because of this, the parallel rays from a distant object focus in front of the retina instead of on it. By placing a concave lens in front of the eye, the incoming parallel rays are diverged slightly so that they appear to come from the far point of the myopic eye, allowing the eye's lens system to focus them perfectly on the retina.
Interesting facts
The far point of a normal eye is at infinity, but for a myopic eye, it recedes to a finite distance (sometimes just a few meters or centimeters).
Question details
Q24 of 36 questions
- Board
- CBSE
- Class
- Class 10
- Subject
- Science
- Year
- 2015
- Section
- Section A
- Marks
- 5
- Type
- Long Answer
- Chapter
- The Human Eye and the Colourful World
- Topic
- Defects of Vision and Their Correction
- Bloom's level
- Applying
- Syllabus topic
- Myopia and its correction
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Frequently asked questions
How many marks does this question carry in CBSE Class 10 Science 2015?
This question carries 5 marks in the CBSE Class 10 Science 2015 examination.
Which chapter does this question come from in Science?
This question is from the chapter "The Human Eye and the Colourful World" in the CBSE Class 10 Science syllabus.
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 2015 paper?
This is a Long Answer question from Section A in the CBSE Class 10 Science 2015 paper.