CBSE-XI-Physics
19: Optical Instruments
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- #Section : i
- Qstn #1Can virtual image be formed on the retina in a seeing process?Ans : The retina acts as a screen; only real images can be obtained on the screen. In case of people having eye defects, the spectacles form the virtual image of the object and the eye lens form the real and inverted image on the retina.
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- Qstn #2Can the image formed by a simple microscope be projected on a screen without using any additional lens or mirror?Ans : The image formed by a simple microscope is virtual and erect. So, it cannot be projected on a screen without using any additional lens or mirror.
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- Qstn #3The angular magnification of a system is less than one. Does it mean that the image formed is inverted?Ans : No, angular magnification is the ratio of the angle subtended by the final image on the eye to the angle subtended by the object on the unaided eye. Its value less than one signifies reduction in the size of the image. It does not mean that the image is inverted.
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- Qstn #4A simple microscope using a single lens often shows coloured image of a white source. Why?Ans : A simple microscope consists of a single convex lens. Sometimes due to chromatic and spherical aberrations, the image of a white source seems coloured at the corners of the lens and somewhere in between.
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- Qstn #5A magnifying glass is a converging lens placed close to the eye. A farsighted person uses spectacles having converging lenses. Compare the functions of a converging lens used as a magnifying glass and as spectacles.Ans : A converging lens in a magnifying glass is of small focal length which is used to magnify an object which is placed close to the lens. On the other hand, converging lens used as spectacles is of varying focal length which depends upon the actual near point of the long-sighted person. It forms image at the near point of the defected eye which is further focussed by the eye lens.
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- Qstn #6A person is viewing an extended object. If a converging lens is placed in front of his eyes, will he feel that the size has increased?Ans : If a person views an extended object through a converging lens, then it will appear larger and wider to him.
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- Qstn #7The magnifying power of a converging lens used as a simple microscope is
1+Df.A compound microscope is a combination of two such converging lenses. Why don’t we have magnifying power
1+Dfo1+Dfe?In other words, why can the objective not be treated as a simple microscope but the eyepiece can?Ans : In a simple microscope, the converging lens is used to magnify the object. It is done by the eyepiece in a compound microscope. But the purpose of the objective lens is the same, i.e., to form an enlarged, real and inverted image of the object at a distance less than the focal length of the eyepiece. So, its magnification power cannot be expressed in a way it is expressed for a simple microscope.
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- Qstn #8By mistake, an eye surgeon puts a concave lens in place of the lens in the eye after a cataract operation. Will the patient be able to see clearly any object placed at any distance?Ans : The image formed by a concave lens is virtual and upright. It is smaller than the object and is formed between the object and the lens, irrespective of the position of the object. If, by mistake, an eye surgeon puts a concave lens in place of eye lens in a patient's eye, then the image will not be focused on the retina; this will lead to unclear vision.
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- Qstn #9The magnifying power of a simple microscope is given by
1+Df,where D is the least distance for clear vision. For farsighted persons, D is greater than the usual. Does it mean that the magnifying power of a simple microscope is greater for a farsighted person as compared to a normal person? Does it mean that a farsighted person can see an insect more clearly under a microscope than a normal person?Ans : The magnifying power of a simple microscope depends on the ratio `` \frac{D}{f}`` for a farsighted person. Here, D for a farsighted person is greater than that for a normal person, but the value of f remains the same. Therefore, the magnifying power of a simple microscope is greater for a farsighted person compared to that for a person with normal vision. Also, a farsighted person can see the insect more clearly under the microscope than a person with normal vision.
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- Qstn #10Why are the magnification properties of microscopes and telescopes defined in terms of the ratio of angles and not in terms of the ratio of sizes of objects and images?Ans : Instruments like telescopes and microscopes deal with objects placed at different distances. Due to some physical factors, there is a relative change in heights not in the angle which the light emerging from them subtends on the lens. So, the magnification properties of instruments are defined in terms of the ratio of angles.
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- Qstn #11An object is placed at a distance of 30 cm from a converging lens of focal length 15 cm. A normal eye (near point 25 cm, far point infinity) is placed close to the lens on the other side.Ans : Object distance, u
= `` -``30 cm
Focal length, f = 15 cm
Image distance, v = ?
The lens formula is given by
`` \frac{1}{v}-\frac{1}{u}=\frac{1}{f}``
`` \Rightarrow \frac{1}{v}-\frac{1}{-30}=\frac{1}{15}``
`` \Rightarrow \frac{1}{v}=\frac{1}{30}``
`` \Rightarrow v=+30\,\mathrm{\,cm\,}``
(on the opposite side of the object)
- #11-aCan the eye see the object clearly?Ans : No, the eye placed close to the lens cannot see the object clearly.
- #11-bWhat should be the minimum separation between the lens and the eye so that the eye can clearly see the object?Ans : The eye should be 30 cm away from the lens to see the object clearly.