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Microscope is an optical instrument used to see very small objects. It’s magnifying power is given by

Simple microscope

  • It is a single convex lens of lesser focal length.
  • Also called magnifying glass or reading lens.
  • Magnification when final image is formed at D and ∞ (i.e., mD and m).
    113586.png and 113595.png
  • If lens is kept at a distance a from the eye, then 113602.png and 113608.png
Fig. 35

Compound microscope

Fig. 36
  • Consists of two converging lenses called objective and eye lens.
  • feye lens > fobjective and (diameter)eye lens > (diameter)objective
  • Intermediate image is real and enlarged.
  • Final image is magnified, virtual and inverted.
  • uo = distance of object from objective (o), vo = distance of image (A’B’) formed by objective from objective, ue = distance of A’B’ from eye lens, ve = distance of final image from eye lens, fo = focal length of objective, fe = focal length of eye lens.

Final image is formed at D

Magnification 113618.png and length of the microscope tube (distance between two lenses) is LD = vo + ue.
Generally, object is placed very near to the principal focus of the objective. Hence, uo  fo. The eye piece is also of small focal length and the image formed by the objective is also very near to the eye piece. So vo LD, the length of the tube.
Hence, we can write 113638.png

Final image is formed at ∞ Magnification

113644.pngand length of tube L = v0 + fe
In terms of length 113654.png
  • For large magnification of the compound microscope, both fo and fe should be small.
  • If the length of the tube of microscope increases, then its magnifying power increases.
  • The magnifying power of the compound microscope may be expressed as M = mo × me, where mo is the magnification of the objective and me is magnifying power of eye piece.

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