Chapter 10 · The Human Eye and the Colourful World

NCERT Class 10 Science Chapter 10 Intext Questions (Page 164)

All 4 questions of NCERT Class 10 Science Chapter 10 Intext Questions (Page 164) — The Human Eye and the Colourful World — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   What ​is meant ​by power ​of ​accommodation ​of ​the eye?
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   It is the ability of the eye lens to change its focal length so that objects at different distances focus on the retina.

   The power of accommodation of the eye is the ability of the eye lens to adjust its own focal length, so that objects lying at different distances are all focused sharply on the retina. The eye lens is made of a fibrous, jelly-like material whose curvature is changed by the ciliary muscles: when these muscles relax the lens becomes thin and its focal length increases, which lets distant objects be seen clearly, and when they contract the lens becomes thicker and its focal length decreases, which lets nearby objects be seen clearly. This adjustment is necessary because the retina is fixed about $2.5$ cm behind the eye lens, so the image distance cannot change and only the focal length can. The ability is limited, because the focal length cannot be reduced below a certain minimum; that is why a normal eye can see clearly only from about $25$ cm, its near point, out to infinity, its far point.

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2. Q2
   A person with a myopic eye ​cannot see objects ​beyond $1.2$ m ​distinctly. ​What should be the ​type of the corrective lens used ​to restore proper vision?
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   A concave (diverging) lens of focal length $-1.2$ m and power $-0.83$ D is needed.

   A concave lens, that is a diverging lens, should be used. Since the person cannot see beyond $1.2$ m, the far point of the eye is $1.2$ m in front of it instead of infinity. The corrective lens must form the image of an object at infinity exactly at that far point, so $u=-\infty$, giving $\frac{1}{u}=0$, and $v=-1.2$ m. The lens formula $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$ then gives $\frac{1}{f}=-\frac{1}{1.2}$, so $f=-1.2$ m. The focal length is negative, which is the mark of a concave lens, and its power is $P=\frac{1}{f}=\frac{1}{-1.2}=-0.83$ D.

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3. Q3
   What ​is the ​far point ​and near ​point ​of the human eye ​with normal ​vision?
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   For a normal eye the far point is at infinity and the near point is about 25 cm.

   For a human eye with normal vision the far point is at infinity and the near point is about $25$ cm from the eye. The far point is the farthest point up to which the eye can see objects clearly, and a normal eye has no such limit, so it lies at infinity. The near point is the minimum distance at which an object can be seen most distinctly and without strain, also called the least distance of distinct vision; for a young adult with normal vision it is about $25$ cm. A normal eye can therefore see clearly everything lying between $25$ cm and infinity.

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4. Q4
   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?
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   The defect is myopia, corrected with spectacles having a concave lens of suitable power.

   The child is most likely suffering from myopia, that is near-sightedness or short-sightedness. The blackboard seen from the last row is a distant object, and a myopic eye can see nearby objects clearly but cannot see distant objects distinctly, because its far point has come nearer than infinity and may be only a few metres away. In such an eye the image of a distant object is formed in front of the retina instead of on it, either because the eye lens is too strongly curved or because the eyeball has become too long, and the blackboard therefore looks blurred. It can be corrected by wearing spectacles fitted with a concave, that is diverging, lens of suitable power. The concave lens spreads the parallel rays coming from the blackboard so that the eye's own lens now brings them to a focus on the retina; equivalently, it forms the image of the distant blackboard at the child's far point, which is a distance the eye can still focus.

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