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Answer the following questions: (e) Wh...

Answer the following questions:
(e) When viewing through a compound microscope, our eyes should be positioned not on the eyepiece but a short distance away from it for best viewing. Why? How much should be that short distance between the eye and eyepiece?

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(e) The image of the objective in the eye-piece is known as ‘eye-ring’. All the rays from the object refracted by objective go through the eye-ring. Therefore, it is an ideal position for our eyes for viewing. If we place our eyes too close to the eye-piece, we shall not collect much of the light and also reduce our field of view. If we position our eyes on the eye-ring and the area of the pupil of our eye is greater or equal to the area of the eye-ring, our eyes will collect all the light refracted by the objective. The precise location of the eye-ring naturally depends on the separation between the objective and the eye-piece. When you view through a microscope by placing your eyes on one end,the ideal distance between the eyes and eye-piece is usually built-in the design of the instrument.
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The ciliary muscles of eye control the curvature of the lens in the eye and hence can alter the effective focal length of the system. When the muscles are fully relaxed, the focal length is maximum. When the muscles are strained, the curvature of lens increases. That means radius of curvature decreases and focal length decreases. For a clear vision, the image must be on the retina. The image distance is therefore fixed for clear vision and it equals the distance of retina from eye lens. It is about 2.5cm for a grown up person. A perosn can theoretically have clear vision of an object situated at any large distance from the eye. The smallest distance at which a person can clearly see is related to minimum possible focal length. The ciliary muscles are most strained in this position. For an average grown up person, minimum distance of the object should be around 25cm. A person suffering from eye defects uses spectacles (eye glass). The function of lens of spectacles is to form the image of the objects within the range in which the person can see clearly. The image o the spectacle lens becomes object for the eye lens and whose image is formed on the retina. The number of spectacle lens used for th eremedy of eye defect is decided by the power fo the lens required and the number of spectacle lens is equal to the numerical value of the power of lens with sign. For example, if power of the lens required is +3D (converging lens of focal length 100//3cm ), then number of lens will be +3 . For all the calculations required, you can use the lens formula and lensmaker's formula. Assume that the eye lens is equiconvex lens. Neglect the distance between the eye lens and the spectacle lens. Q. Maximum focal length of a eye lens of a normal person is

NCERT GUJARATI-RAY OPTICS AND OPTICAL INSTRUMENTS-EXERCISES
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  5. (a) Determine the 'effective focal length' of the combination of the t...

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  7. A card sheet divided into squares each of size 1 mm^(2) is being viewe...

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  8. (a) At what distance should the lens be held from the figure in Exerci...

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  9. What should be the distance between the object in Exercise 9.24 and th...

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  10. Answer the following questions: (a) The angle subtended at the eye b...

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  11. Answer the following questions: (b) In viewing through a magnifying ...

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  12. Answer the following questions: (c) Magnifying power of a simple mic...

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  13. Answer the following questions: (d) Why must both the objective and ...

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  14. Answer the following questions: (e) When viewing through a compound ...

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  15. An angular magnification (magnifying power) of 30X is desired using an...

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  16. A small telescope has an objective lens of focal length 140cm and an e...

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  17. (a) For the telescope described in Exercise 9.28 (a), what is the sepa...

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  19. Light incident normally on a plane mirror attached to a galvanometer c...

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  20. Figure 9.34 shows an equiconvex lens (of refractive index 1.50) in con...

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