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i) Draw a neat labelled ray diagram of a compound microscope. Explain briefly its working. (ii) Why must both the objective and the eyepiece of a compound microscope have short focal lengths ? |
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Answer» Solution :(i) A compound microscope consists of an objective lens O of small focal length and small aperture and an eyepiece E of small focal length but slightly greater aperture as shown in Fig. 9.84. OBJECT AB is placed at a distance slightly greater than the focal length of objective (`u_(0) gt f_(0)`) so that` A.B.`reaL inverted and magnified IMAGE A.B. is formed at a distance v0 on other side of lens. The image A.B., being situated between principal focus and optical centre of eyepiece, behaves as a real object for eyepiece. As shown in figure the final image A"B" is formed at least distance of distinct vision and is a virtual, inverted and highly magnified one. The MAGNIFYING power of a compound microscope is GIVEN by: `m=-L/f_(0) (1+D/f_(e))`  where L = distance between objective and eyepiece lenses, D = least distance of distinct vision and `f_(0)`and `f_(e)`are the focal lengths of objective and eyepiece lenses respectively. (ii) To have higher magnifying power a compound microscope must have both the objective and the eyepiece having SHORT focal length. This fact is clear from the formula for magnifying power of microscope given above. |
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