Those instrument which uses lens or mirror and form the images according to requirements are called optical instruments. For examples: camera, microscope, telescope, etc.
Optical instruments are of two types:
Real image forming optical instruments - such as the projector and camera.
Virtual image forming optical instruments - such as microscope, astronomical telescope. The image formed by these instruments is seen directly by the eyes.
A compound microscope has two lenses. One is called eyepiece and the other is called objective lens. The objective lens has smaller focal length than that of the eyepiece.
In a telescope, at first the light rays are collected from objective lens. Then it forms the inverted and real image at its focus. The image formed is adjusted such that the image also lies at the focus of the eyepiece. In other words, it must lie at the focus of both the objective lens and the eye lens. Then the eye lens forms magnified image at infinity. Thus, the telescope helps us to see distant object clearly.
The compound microscopes are used to see small objects in large and magnified form, whereas the telescope is used to see far distant objects. The astronomical telescopes are used to see star, planets, etc. In the space, similarly, the cell, blood or microbes can be seen through the compound microscope.
The two differences between compound microscope and astronomical microscope are as follows:
Compound microscope Astronomical microscope
It is used to observe microscope objects. It is used to see far astronomical objects.
In compound microscope, focal length of objective lens is smaller than eye lens. In telescope, focal length of objective lens is long while focal length of eye lens is short.
Lenses are kept in such a manner that the image formed by the objective lens falls within the focus of eye lens. Two lens are kept in such a manner that both of the lens should have the same principal focus.
The image of an object made by an objective lens in compound microscope lies always in same point. Therefore, in microscope, it is not necessary to change the distance between the objective lens and the eye lens.
But while using telescope, the distance of an object is changed. In this way, objective lens forms the image of an object at different distances at different places. Therefore to place the image on the focus, the distance between the objective lens and the eye lens should be changed.
The objective lens of a telescope should collect scattered light rays on the large area to observe the distant object distinctly. Therefore, the objective lens of telescope is made larger than that of compound microscope.