As a simple microscope, a convex lens is satisfactory for magnifying small nearby objects up to about twenty times their original size. For large magnification, a compound microscope is used, which is a combination of two lenses.
Near point is the distance from the eye for which the image of an object placed there is formed (by eye lens) on the retina. The near point varies from person to person and with the age of an individual. At a young age (below 10 years), the near point may be as close as 7-8 cm. In the old age, the near point shifts to larger values, say 100-200 cm or even more. That is why young children tend to keep their books so close whereas the aged persons keep a book or newspaper far away from the eye.
Least Distance of Distinct Vision
Least distance of distinct vision is the distance up to which the human eye can see the object clearly without any strain on it. For a normal human eye, this distance is generally taken to be 25 cm.
Angular magnification is the ratio of the angle subtended by the image at the eye (when the microscope is used) to the angle subtended by the object at the unaided eye when the object is placed at the least distance of distinct vision. It is also called the magnifying power of the microscope.
When the image is formed at infinity, least strain is exerted on the eye for getting it focused on the retina. This is known as normal adjustment.
Linear magnification is the ratio of the size of the image to the size of the object.
Visual angle is the angle subtended by the object at human eye.