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Draw the ray diagram for simple microscope. Write expression for its magnifying power. |
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Answer» Solution :Microscope. Microscope is a device which is used to see very small objects which cannot be seen with naked eye. Simple Microscope Principle. When an OBJECT is placed between principal focus and optical centre of a convex lens, a vertical and erect image will be formed on the same side of the object. Construction and Action An ordinary convex lens of small focal length held close to the eye can be used as magnifying glass or a simple microscope. The object is made to lie between the lens and its focus so that image formed is virtual, erect and magnified. As the object is to be seen clearly, the image is made to lie at the distance of distinct VISION from the eye. The course of rays indicating the formation of image is shown in the figure.  Magnifying Power Magnifying Power of a magnifying glass is defined as the ratio of the angle subtended at the eye by the image to the angle subtended by the object when both are placed at the least distance of distinct vision and seen directly. In Fig., the image AB is fomed at the distance of distinct vision and subtends an `/_A.CB. = beta` on the eye as it is held close to the lens. If the object AB is also moved to the distance of distinct vision i.e., to the position EB., it would SUBTEND an `/_ECB. = alpha` at the eye. Magnifying power, `M=(beta)/(alpha)` `=(tan beta)/(tan alpha)` (`:. beta` and `alpha` are small) `=((AB)/(CB))/((EB.)/(CB.))` `=(AB)/(CB) xx (CB.)/(EB.)` `=(CB.)/(CB)` (`:.` AB = EB.) `M=(v)/(u)` Using the lens formula, `(1)/(v)-(1)/(u)=(1)/(f)` `:. (1)/(u)=(1)/(v)-(1)/(f)` or `(v)/(u)=(v)/(v)-(v)/(f)` or `(v)/(u)=1-(v)/(f)` Since image is formed at least distance of distinct vision D and is virtual, `:.`v = - D So `M=1-(-D)/(f)` or `M=1+(D)/(f)` This shows that SMALLER the focal length of the lens, greater will be the magnifying power of microscope. |
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