Simple Microscope or Magnifying Glass and its Magnification

An optical instrument, which is used to see small objects distinctly is called Simple Microscope or Magnifying glass.  

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      A convex lens which help us to observe clearly an object (very small object) which is brought close to eye i.e. within least distance of distinct vision, is called Magnifying Glass.

When the object is placed inside the focal point of the lens, then erect, magnified and virtual image is formed at near point or 25 cm.

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      The angular magnification of the magnifying glass is defined as the ratio of visual angle subtended by the image seen through a magnifying glass to the visual angle subtended by the object when placed at the least distance of distance of distinct vision and seen through naked eye i.e.

                                   

    Now suppose a small object OP forms a visual angle "", when it is placed at least distance of vision and when it is placed at distance "p" within focal length of magnifying glass then its erect, virtual and magnified image IQ is formed at least distance of vision and it subtends a visual angle " " as shown in the figures below:

  Fig A    Object Angle

  Fig B    Image Angle

 In figure A, the right angle triangle ∆OAP gives,

For small angle  

Now from figure B, the right angle triangle ∆OAP gives,

putting the values of and in equation (1), we get

or,

According to the lens formula,

As the final image is virtual then,

Multiplying both sides by "d" ,

since ,

or,

    This expression gives magnifying power of magnifying glass or simple microscope.

      It is thus obvious that for a lens of high angular magnification the focal length should be small. If, for example, f = 5 cm and d = 25 cm, then M = 5, the object would look five times larger when viewed through such a lens.