working principle of compound microscope

Features of Compound Microscope

A compound microscope is an optical instrument which is used to magnify very small objects like blood cells, bacteria which otherwise cannot be seen with the naked eye.

A Laboratory Model of Compound Microscope

The essential parts of a compound microscope are two convex lenses of short focal length. These lenses are referred to as:

• the objective lens or objective

• the eye piece or lens.

Construction of a Compound Microscope

A compound microscope consists of the following parts:

• Objective lens

The objective lens of a compound microscope is a convex lens of very short focal length (f_o) that is f_o < 1cm. The object to be seen is kept very close to the objective lens.

• Eye piece

The eye piece of a compound microscope is also a convex lens of short focal length f_e. But f_e > f_o.

• Microscope tube

The objective lens and the eyepiece are mounted coaxially (having a common axis) at the ends of two brass tubes which can be made to slide into each other so that the distance between the two lenses can be adjusted.

Working

The ray diagram given below gives the principle of a compound microscope. The object is mounted on the stand below the microscope tube. The objective lens forms a real, inverted and magnified image (I₁) of the object. The image I₁ acts as an object for the eye piece. The position of the eyepiece is so adjusted that the image lies within the focus of the eyepiece (F_e). The eyepiece acts like a magnifying glass and forms a virtual erect and magnified image of the object.

Image Formation in a Compound Microscope

• The object (O) is placed just outside F_o, the principal focus of the objective lens.

• F_e is the principal focus of the eye lens.

• A real, inverted magnified image I₁ is formed. The magnified image I₁ acts as an object for the eye lens.

• The final image I₂ is virtual and is magnified still further. It is inverted compared with the object. I₂ may appear 1000 times larger than the object.

Magnifying Power of a Compound Microscope                                      

       The magnifying power of a compound microscope is defined as the ratio of the size of the final image (I₂) as seen through the microscope to the size of the object as seen with a naked eye.

Image Formation in a Compound Microscope

= m_objective x m_eyepiece

= m_o x m_e

Where m_objective (m_o) and m_eyepiece (m_e) are the magnification produced by the objective and eyepiece respectively.

m = m_o x m_e

Eye piece is nothing but a simple microscope

The lens formula is

But distance between the object and the lens is -u.

Multiply equation (2) by V

m = m_o x m_e