A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
The child 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 student is sitting in the last row and has difficulty reading the blackboard, it indicates inability to see distant objects clearly.
In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. This defect may arise due to excessive curvature of the eye lens or elongation of the eyeball.
This defect can be corrected by using a concave lens of suitable power. A concave lens will bring the image back onto the retina and thus the defect is corrected.
Explanation
The question describes a classic case of myopia from the NCERT textbook. The key indicators are: (1) difficulty seeing the blackboard from the last row = difficulty seeing distant objects, and (2) the context explicitly states that myopia means 'can see nearby objects clearly but cannot see distant objects distinctly.' The correction method is clearly stated in the text - using a concave lens of suitable power.
Solution Steps
Step 1: Identify the symptom - difficulty seeing distant objects (blackboard from last row)
Step 2: Match with defect - Myopia (near-sightedness) where distant objects cannot be seen distinctly
Step 3: State the cause - Image forms in front of retina due to excessive curvature of eye lens or elongation of eyeball
Step 4: Provide correction - Use concave lens of suitable power to bring image back onto retina