The size of the image on the retina of the observer depends on

i) the real size of the object, h

ii) the distance of the object from the observer, u.

These two factors (h and u), determine the size of the angle subtended at the eye by the object ( in the above diagrams).

The apparent size of an object is proportional to this angle.

Optical instruments provide magnification by increasing the size of the angle subtended at the eye.

Angular magnification, M, is defined as

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