# Physics Class 12 NCERT Solutions: Chapter 9 Ray Optics and Optical Instruments Part 12 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 25. Does short-sightedness (myopia) or long-sightedness (hypermetropia) imply necessarily that the eye has partially lost its ability of accommodation? If not, what might cause these defects of vision?

A myopic or hypermetropic person can also possess the normal ability of accommodation of the eye-lens. Myopia occurs when the eye-balls get elongated from front to back. Hypermetropia occurs when the eye-balls get shortened. When the eye-lens loses its ability of accommodation, the defect is called presbyopia.

Q: 26. A myopic person has been using spectacles of power dioptre for distant vision. During old age he also needs to use separate reading glass of power dioptres. Explain what may have happened.

The power of the spectacles used by the myopic person,

Focal length of the spectacles,

Hence, the far point of the person is . He might have a normal near point of . When he uses the spectacles, the objects placed at infinity produce virtual images at . He uses the ability of accommodation of the eye-lens to see the objects placed between and .

During old age, the person uses reading glasses of power,

The ability of accommodation is lost in old age. This defect is called presbyopia. As a result, he is unable to see clearly the objects placed at .

Q: 27. A person looking at a person wearing a shirt with a pattern comprising vertical and horizontal lines is able to see the vertical lines more distinctly than the horizontal ones. What is this defect due to? How is such a defect of vision corrected?

In the given case, the person is able to see vertical lines more distinctly than horizontal lines. This means that the refracting system (cornea and eye-lens) of the eye is not working in the same way in different planes. This defect is called astigmatism. The person՚s eye has enough curvature in the vertical plane. However, the curvature in the horizontal plane is insufficient. Hence, sharp images of the vertical lines are formed on the retina, but horizontal lines appear blurred. This defect can be corrected by using cylindrical lenses.

Q: 28. A man with normal near point reads a book with small print using a magnifying glass: a thin convex lens of focal length .

(A) What is the closest and the farthest distance at which he should keep the lens from the page so that he can read the book when viewing through the magnifying glass?

(B) What is the maximum and the minimum angular magnification (magnifying power) possible using the above simple microscope?

(A) Focal length of the magnifying glass,

Least distance of distance vision,

Closest object distance

Image distance,

According to the lens formula, we have:

Hence, the closest distance at which the person can read the book is .

For the object at the farthest distant , the image distance

According to the lens formula, we have:

Hence, the farthest distance at which the person can read the book is .

(B) Maximum angular magnification is given by the relation

Minimum angular magnification is given by the relation:

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