Least Distance of distinct vision (D) :
The closest distance that the 'normal' human eye can observe
clearly; without any strain to the eye is called the least distance of distinct
In connection with optical instruments the object would be
considered to be held whenever feasible at the least distance of distinct vision
for determining its apparent size even if the actual position of the object
may not be at (D). Usually D = 25 cm.
Magnifying Power (M) of an Optical Instrument :
It is defined as the ratio of the apparent size of the final
image i.e. angle subtended by the image at the position of eye (b)
to the apparent size of the object i.e. angle subtended by the object at the
Simple Microscope (Magnifying glass) and its Magnifying
Power (M) :
The property of a convex lens to produce a virtual, erect and
magnified image on the same side of the lens as the object allows the convex
lens to be used as an optical instrument called simple microscope.