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COMPOUND
MICROSCOPE
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| Compound microscope is an optical instrument which is used to obtain high magnification. | |||
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Construction
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It
consists of two converging lenses:
Objective
Eye piece |
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Objective
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| The lens in front of object is called objective. Its focal length f1= fo is taken to be very small .The objective forms a real, inverted, and magnified image of the object placed just beyond the focus of objective. | |||
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Eye
piece
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| The lens towards the observer's eye is called piece .Focal length of eye piece is greater than the focal length of objective. Eye piece works as a magnifying glass. | |||
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Working
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| The
objective is so adjusted that the object is very closed to its focus. The
objective forms a real , inverted and magnified image of the abject beyond 2fo on the right hand side. The eye piece is so adjusted that it forms a virtual image at the least distance of distinct vision "d" .The final image is highly magnified. |
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Magnifying
power
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| In
order to determine the magnifying power of a compound microscope ,we consider
an object oo' placed in front of objective at a
distance p1. Objective forms an inverted image II' at
a distance of q1 from the objective. Magnification produced by the objective is given by: |
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Mo=
size of image / size of object
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Mo=
q1/ p1--------------- (1)
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| Eye piece works as a magnifying glass. It further magnifies the first image formed by objective. | |||
| Magnification produced by the eye piece is given by: | |||
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Me=
size of image / size of object
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Me=
q2/ p2
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| We know that the eye piece behaves as a magnifying glass therefore the final image will be formed at least distance of distinct vision i.e at 25 cm from the eye. Hence q2 = d | |||
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Me=
d / p2--------------- (2)
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| Using thin lens formula for eye piece : | |||
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1/f2
= 1/q2 + 1/p2
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| Here f2 = fe, q2 = - d and p = p2 | |||
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1/fe
= 1/-d + 1/p2
1/fe = -1/d + 1/p2 Multiplying both sides by "d" d/fe = -d/d + d/p2 d/fe = -1 + d/p2 1 + d/fe = d/p2 d/p2 = 1 + d/fe----------------(3) Comparing equation (2) and (3) Me = 1 + d/fe--------(4) |
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| Total
magnification is equal to the product of the magnification produced by the
objective and the eye piece. |
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M =Mo X
Me
M = (q1/p1)(1 + d/fe) |
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| In
order to get maximum magnification, we must decrease p1 and increase
q1 .Thus maximum possible value of p1 is fo i.e p = fo and maximum possible value of q1 is the length of microscope i.e q1 = L Therefore the magnification produced by a compound d microscope is given by: |
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M
= (L/fO)(1
+ d/fe)
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