A Myopic Person Has Been Using Spectacles of Power −1.0 Dioptre for Distant Vision. During Old Age He Also Needs to Use Separate Reading Glass of Power + 2.0 Dioptres. Explain What May Have Happened. - Physics


A myopic person has been using spectacles of power −1.0 dioptre for distant vision. During old age, he also needs to use the separate reading glass of power + 2.0 dioptres. Explain what may have happened.



The power of the spectacles used by the myopic person, P = −1.0 D

Focal length of the spectacles, `f = 1/P = 1/(-1xx10^(-2)) = -100 cm`

Hence, the far point of the person is 100 cm. He might have a normal near point of 25 cm. When he uses the spectacles, the objects placed at infinity produce virtual images at 100 cm. He uses the ability of accommodation of the eye-lens to see the objects placed between 100 cm and 25 cm.

During old age, the person uses reading glasses of power, `P' = +2D`

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 25 cm.

Concept: Optical Instruments: the Eye
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Chapter 9: Ray Optics and Optical Instruments - Exercise [Page 348]


NCERT Physics Class 12
Chapter 9 Ray Optics and Optical Instruments
Exercise | Q 26 | Page 348


What should be the distance between the object and the magnifying glass if the virtual image of each square in the figure is to have an area of 6.25 mm2. Would you be able to see the squares distinctly with your eyes very close to the magnifier?

In viewing through a magnifying glass, one usually positions one’s eyes very close to the lens. Does angular magnification change if the eye is moved back?

What is the height of the final image of the tower if it is formed at 25 cm?

The muscles of a normal eye are least strained when the eye is focussed on an object

When objects at different distances are seen by the eye, which of the following remain constant?

A person A can clearly see objects between 25 cm and 200 cm. Which of the following may represent the range of clear vision for a person B having muscles stronger than A, but all other parameters of eye identical to that of A?

The focal length of a normal eye-lens is about

The distance of the eye-lens from the retina is x. For a normal eye, the maximum focal length of the eye-lens

A man wearing glasses of focal length +1 m cannot clearly see beyond 1 m

When we see an object, the image formed on the retina is
(a) real
(b) virtual
(c) erect
(d) inverted

Mark the correct options.
(a) If the far point goes ahead, the power of the divergent lens should be reduced.
(b) If the near point goes ahead, the power of the convergent lens should be reduced.
(c) If the far point is 1 m away from the eye, divergent lens should be used.
(d) If the near point is 1 m away from the eye, divergent lens should be used.

A person looks at different trees in an open space with the following details. Arrange the trees in decreasing order of their apparent sizes.

Tree Height(m) Distance from the eye(m)
A 2.0 50
B 2.5 80
C 1.8 70
D 2.8 100


Can virtual image be formed on the retina in a seeing process?

A normal eye has retina 2 cm behind the eye-lens. What is the power of the eye-lens when the eye is (a) fully relaxed, (b) most strained?


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