11th Class Biology The Living World / जीव जगत Microscopy


Category : 11th Class

Microscopy (Gk. Micros = small ; skopein = to see) It is practice of using microscopes for the study of finer details of small objects including cells and tissues. Microscope are instruments consisting of lenses (made of glass / Lithium fluoride / electromagnetic lens) which magnify and resolve small objects not visible to unaided eye for the study of their details. The term microscope was coined by Faber in 1625.

Magnification : Is the power of enlargement, which is the ratio of

\[\text{Magnification}=\frac{\text{Size of the image with}\,\text{the instrument}}{\text{Size of the image with unaided eye}}\]

Magnification of a microscope is roughly equal to the multiple of magnifying power of objective lens and ocular lens (eye piece) e.g., if the magnification power of an ocular lens is \[10\,X\] and of the objective is \[40\,X,\] then the total magnifying power of a microscope is \[10\times 40=400\,X\] (the magnification power of a microscope is represented by the symbol 'X').

Resolving power : It is the ability of a system to distinguish two close objects as two distinct objects. Its values is calculated by Abbe equation -

\[{{L}_{m}}=\frac{0.61\lambda }{NA}\]

Here, \[\lambda -\] is wavelength of used light, \[NA-\] Numerical Aperture, \[(NA=n\sin \theta )\]

Numerical aperture is multiple of refractive index of medium (n) and \[\sin \theta \], which is sine of angle substended by optical axis and outer ray covered by objective. The value for best objective \[\sin e\,70{}^\circ =0.94.\]

The resolving power of human eye is 100mm or microns (0.1 mm). This means that two points less than 100mm apart appear as one point to our eyes.

Father of microscopy is Leeuwenhoek. He built first 270 X magnification microscope in 1672.

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