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Optical Instruments


Optical instruments are simple instruments we use based on the phenomenon of refraction, reflection or both to serve our visual needs.They are classified based on its applications like telescopes, microscopes, sextants that act as visual instruments.

Photographing and projecting instruments like cameras, optical lanterns and epidiascopes. Analyzing and measuring instruments like spectrometer, photometer, etc. It is used for the examination and characterization of matter using visible light. Lets have a simple idea about magnifier, telescopes and microscope in this page.

The human eye is perhaps a natural optical instrument we all have! This marvelous gift has only a single convergent lens. The three main parts of eye are lens, retina and iris as shown in figure.

Human Eye
Eye with its basic parts

The object is imaged by the lens onto back inner surface of the eye called the retina. The retina formed is a real but inverted image of the object. It is amazing that, while images are inverted our brain interprets them as being upright. The iris diaphragm in front of the lens changes its aperture diameter by unconscious muscular action to adapt to the brightness of the environment. It opens wider for the faint light and closes down for bright sunlight. The diameter of iris vary between 1.5 and 7 mm.


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Magnifier is something that makes some thing look bigger. It simply magnifies the image. If you want to see something with finer details you use the convex lens which is known magnifier. If a image is very far it appears tiny and will be close to focal point. To get a magnified image the distance between the object and lens should be less than the focal length. The converging lens acts as a magnifier to sort out this by making it move away from the focal point. Suppose the image is at the focal point 2F moves towards F due to lens the image gets closer that gives out the magnification.

Magnifying Lens
Converging lens as magnifier

The magnifying power is given by
Magnifying Power = $\frac{Size\ of\ the\ image}{Size\ of\ the\ object}$


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Microscope is an instrument used to see the enlarged image of very small objects through our naked eyes or we can say that it is an optical instrument which uses a combination of lenses to produce magnified images of small objects. Basically they are of two types
  • Optical microscope
  • Electron microscope
Optical or Light microscope is so called because it uses a light source to view the enlarged image of sample objects.It is probably most well-known and well-used tool in laboratories whereas electron microscope uses a highly energetic electron beam instead of light ray to examine the sample objects.

Compound Microscope

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A compound microscope is an optical instrument, which is used to see the very tiny objects. It consists of the two lenses which are connected to the two ends of a two tubes. These both lenses are convex in nature. One of the lenses is called the objective lens and the other is called the eyepiece. The figure below shows it
Compound Microscope
Compound microscope construction


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If we are going for astronomical observation the eye has limited sensitivity. A distant source of light such as a star will not be seen at all unless the intensity of light from it reaches the eye that distinguishes the fine detail has limitation by retina due to small aperture of eye. Our human has also got limited resolution. The invention of telescope by Hans Lipperhey overcame these short comings. It is used for viewing distant objects with the greater resolving power between the distant objects. They are of two types
  • Refracting telescope
  • Reflecting telescope