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An object can be made to appear ‘big’ or ‘magnified’ by increasing the visual angle. Magnification or magnifying power (m) is the ratio of the visual angle while looking through the instrument to that with the naked eye.
For small angles, magnification is defined as Description: 83375.png or Description: 83382.png.
Some of the commonly used optical instruments are given below.


Description: Description: 89185.png

Ray diagrams of the above-mentioned optical instruments are shown in Figures .


Description: Description: 89217.png

Ray Diagram of Simple Microscope


Magnification Description: 83445.png, where D is the least distance of distinct vision.


Description: Description: 89226.png


Figure 9.28 Ray Diagram of Compound Microscope


Magnification Description: 83486.png, where L is the separation between the objective and the eyepiece D is

the least distance of distinct vision, fo is the focal length of objective and fe is the focal length of eyepiece.


Description: Description: 89334.png
Figure 9.29 Ray Diagram of an Astronomical Telescope


Magnification m = Description: 89852.png, where fo is the focal length of the objective and fe is the focal length of the eyepiece.
Differences Between Compound Microscope and Astronomical Telescope

Compound Microscope

Astronomical Telescope

• Objective of shorter focal length

• Eyepiece of larger focal length

• Object, outside f0 (principal focus of objective)

• Used for studying microorganisms (to enlarge very small objects)

• Objective of larger focal length

• Eyepiece of shorter focal length

• Object at infinity

• Used for astronomical purposes (to view distant objects)

A pair of telescopes, which is designed to be used together is called a binocular.

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