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Telescope: Principle, Different Parts, and Advantages

Class 7
Jun 12, 2023


A lens is an object made of transparent material having one or both sides curved.

As light moves in a straight line, a lens bend the light and make it reach our eyes and create an image in it. The image of the object seen through the lens can look bigger or smaller than its real size.

Image formation by lenses
Image formation by lenses

A telescope having small lenses can zoom into the stars and other objects in the sky.

Why are we unable to view a distant object clearly?


Why, for instance, is it impossible to see the writing on a coin from 50 feet away with our regular eyes?

The solution is simple: the object doesn’t occupy much room on our eye’s screen (retina).

Our vision is limited when we look at objects. Our eyes can only see clearly up to a certain distance. We must use cameras, magnifying glasses, or telescopes to see an object properly.

Image formation at the eye with and without a telescope
Image formation at the eye with and without a telescope

We will need considerably larger eyes to gather enough brightness and light to see a more detailed image of something at a distance to see the image more clearly; you would then need to magnify it.



The Main Parts of a Telescope:

  1. The first component is an objective lens; it collects a lot of light from a far-off object in the distance and brings that light and the image to focus.
  2. The bright light will then be magnified by an eyepiece lens, expanding outward, which can be seen clearly with our own eyes.
Main parts of a telescope
Main parts of a telescope

The Main Principle of a Telescope:

The basic concept is to gather a lot of light to create a bright image within the telescope and then magnify (enlarge) that bright image so that it occupies a large portion of your retina.

The Main Characteristics of a Telescope:

  • Its ability to collect light: The diameter of the lens or mirror, the aperture used to gather light, determines how well a telescope can collect light.
  • Its ability to magnify a picture: The combination of lenses the telescope uses determines its magnification, or capacity to magnify a picture.

Mainly Telescopes are of Two Types:

Two types of Telescope
Two types of Telescope

Refracting Telescope:

This was originally developed in astronomy by Galileo. The designs of Galileo combined convex and concave lenses. Kepler refined the design in the year 1611 by adding two convex lenses, which turned the image around.

The most common form of a telescope is the refracting telescope.

Refracting Telescope
Refracting Telescope

The parts of a refracting telescope are as follows:

  • A large tube built of wood, plastic, or metal
  • A front glass lens (objective lens)
  • A second glass lens (eyepiece)

The tube: It maintains the lenses’ proper spacing from one another. The tube also helps in blocking out light, moisture, dust, and other elements that could obstruct the formation of a clear image.

The objective lens: It collects the light and focuses it by bending or refracting it.

The eyepiece: It brings the image to your eye and magnifies the image. Eyepieces have much shorter focal lengths than objective lenses.

Disadvantages of Using a Refracting Telescope:

1. Chromatic Aberration:

This is a problem when using lenses. Different wavelengths of light refract in different amounts.

Chromatic Aberration
Chromatic Aberration
    1. The result of this is that the edges of the image are blurred and colored like fuzzy rainbows. One can get special lenses with a special coating to reduce the amount of chromatic aberration, but they are expensive.
    2. The size of a refracting telescope is generally very huge and difficult to mount while using.
    3. The brightness of the image gets reduced due to absorption by the lens.

Reflecting Telescope:

In comparison to a refractor telescope, which relies on a lens to gather light, reflecting telescopes employ mirrors to do so. The mirror of a reflector is located at the telescope tube’s bottom end, eliminating the difficulties associated with supporting a lens in a refractor telescope as well as the light losses caused by light passing through the heavy glass.

Refracting and reflecting telescope
Refracting and reflecting telescope

A reflecting telescope employ mirrors to reflect the light within. Reflecting telescopes are somewhat more complicated than refracting telescopes.

Reflecting Telescope
Reflecting Telescope

The light enters at one end and is reflected by a concave primary mirror at the other end into a secondary mirror, which is a smaller mirror that is typically mounted on the side of the telescope. The light is reflected by the secondary mirror and enters an eyepiece, which is normally positioned on the side of the telescope and has a focus-adjustable eyepiece.

Advantages of Reflecting Telescopes
  1. Reflecting telescopes are not affected by chromatic aberration because all wavelengths of light get reflected by the mirror uniformly since the reflected light does not scatter according to wavelength.
  2. A reflector’s telescope tube is small than a refractor of the same diameter, which lowers the cost of the tube and makes them simpler and less expensive. They are also simpler to install because the mount can be attached to the mirror’s back.
  3. Reflector telescopes are often more affordable since mirrors are cheaper to build than lenses.


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