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| Compound Microscope |
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| A compound microscope is an optical instrument which is used to magnify very small objects like blood cells, bacteria which otherwise cannot be seen with the naked eye. |
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| A Laboratory Model of Compound Microscope |
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| The essential parts of a compound microscope are two convex lenses of short focal length. These lenses are referred to as: |
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 the objective lens or objective |
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 the eye piece or lens |
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| A compound microscope consists of the following parts: |
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 Objective lens |
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| The objective lens of a compound microscope is a convex lens of very short focal length (fo) that is fo < 1cm. The object to be seen is kept very close to the objective lens. |
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 Eye piece |
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| The eye piece of a compound microscope is also a convex lens of short focal length fe. But fe > fo. |
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 Microscope tube |
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| The objective lens and the eyepiece are mounted coaxially (having a common axis) at the ends of two brass tubes which can be made to slide into each other so that the distance between the two lenses can be adjusted. |
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| The ray diagram given below gives the principle of a compound microscope. The object is mounted on the stand below the microscope tube. The objective lens forms a real, inverted and magnified image (I1) of the object. The image I1 acts as an object for the eye piece. The position of the eyepiece is so adjusted that the image lies within the focus of the eyepiece (Fe). The eyepiece acts like a magnifying glass and forms a virtual erect and magnified image of the object. |
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| Image Formation in a Compound Microscope |
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 The object (O) is placed just outside Fo, the principal focus of the objective lens. |
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 Fe is the principal focus of the eye lens. |
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 A real, inverted magnified image I1 is formed. The magnified image I1 acts
as an object for the eye lens. |
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 The final image I2 is virtual and
is magnified still further. It is inverted compared with the object. I2 may appear 1000 times larger than the object. |
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| The magnifying power of a compound microscope is defined as the ratio of the size of the final image (I2) as seen through the microscope to the size of the object as seen with a naked eye. |
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| Image Formation in a Compound Microscope |
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| = mobjective x meyepiece |
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| = mo x me |
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| Where mobjective (mo) and meyepiece (me) are the magnification produced by the objective and eyepiece respectively. |
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| m = mo x me |
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| Eye piece is nothing but a simple microscope |
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| The lens formula is |
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| But distance between the object and the lens is -u. |
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| Multiply equation (2) by V |
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| m = mo x me |
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