Compound Microscope

A compound microscope consists of two convex lenses. A lens of short aperture and short focal length faces the object and is called the objective. Another lens of short focal length but large aperture facing the eye is called the eye piece. The objective and eye piece are placed co-axially at the two ends of a tube.

When the object is placed between F and 2F of the objective, its a real, inverted and magnified image is formed beyond 2F on the other side of the objective. This image acts as an object for the eye lens, which then acts as a simple microscope. The eye lens is so adjusted that the image lies between its focus and the optical center so as to form a magnified image at the least distance of distinct vision from the eye lens.

Magnifying Power of Compound Microscope

Magnifying power of a compound microscope is defined as the ratio of the angle subtended by the final image at the eye to the angle subtended by the object at unaided eye, when both are placed at the least distance of distinct vision.

M = β/α

Since the angles α and β are small, these can be replaced by their tangents.

M = (tan β)/(tan α)

M = m_o × m_e

M = (L/f_o)(1+D/f_e)