Usually the focal length of the convex lens is around 2.5 cm. The object to be viewed through a simple microscope is placed between the optic center and the focus and the image is erect, virtual and magnified.
The figure below gives the ray diagram showing the principle of the simple microscope. From the ray diagram it is clear that the image formed is erect, virtual and magnified.Magnifying Power of a Simple Microscope
The magnifying power or angular magnification of a microscope may be defined as the ratio of the angle subtended at the eye by the image formed at the distance of the distinct vision to the angle subtended by the object when placed at the distance of the distinct vision.The ray diagram shows that the image of the object AB is formed at A1B1. A1B1 is formed at the least distance of distinct vision.
The figure shows that the angle A1OB1 subtended at the eye by the object in the position A1B1 is greater than the angle AOB subtended by it in the position AB. From this it is clear that the eye estimates the angle subtended by an object on it and not the linear size of the object.
OB = u = distance between the lens and the object
The distance between the image and the lens is negative as the image is virtual.
The lens formula for a convex lens is
Where f is the focal length of the lens
Multiplying both sides of the equation (1) by v we get
From equation (3) it is clear that a convex lens of short focal length has a large magnifying power.
The highest magnification which can be obtained from a simple microscope is about 20.