Telescope: Types, Function, Working & Magnifying Formula

Telescope is a powerful optical instrument that is used to form magnified images of distant objects. Telescopes are often used to view distant objects in space such as planets and stars.

• Telescope has a tubular-shaped structure and it is formed with an arrangement of curved mirrors and lenses.
• Telescopes are based on the principle of refraction as well as reflection.
• Galileo Galilei, an Italian astronomer, was the first person to use a telescope for the study of astronomy.

Astronomical Telescopes are undoubtedly the most important investigation tool in astronomy. They help to collect and analyze radiation from celestial objects, even those in the far reaches of the universe.

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Key Terms: Telescope, Reflecting Telescope, Refracting Telescope, Electromagnetic Radiation, Celestial Bodies, Magnifying Power, Hubble Space Telescope

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What is Telescope?

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Telescope is an optical instrument that has a particular arrangement of curved mirrors and lenses in such a manner that distant objects appear nearer. 

• Distant objects can be seen through telescopes through their emission, absorption, or reflection of electromagnetic radiation.
• Telescopes are one of the most important tools in astronomy. 
• They are used in the study of planets, stars, and other objects in outer space.
• There are two major types of telescopes namely Refracting Telescopes and Reflecting Telescopes. 
• Telescopes are used for different regions of the electromagnetic spectrum and other types of detectors.

Telescope

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Working of Telescope

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Telescopes work by collecting as much light as possible with the help of curved mirrors or lenses through a large aperture which is much more than our eyes. Since they collect so much light, we can see the objects invisible to the naked eye with their help.

• The big lens is used to collect the light from the source.
• The small lens brings the image of that object to our eyes.

Working of Telescope

Working Principle of Telescope 

Telescopes work on the principle that in the case when the object that has to be magnified is placed at a very large distance from the objective lens of the telescope, the image formed is:

• Virtual
• Inverted
• Magnified

This image is formed at the least distance of distinct vision from the eye that is placed close to the eyepiece.

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Parts of Telescope

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The components or parts of a telescope are as follows: 

• Optical Tube: It holds the mirrors and lenses of a telescope. These mirrors or lenses gather light from distant objects during nighttime and form a clear image for the viewer.
• Eyepiece: It forms the magnified image of the object and passes it to the eye of the viewer.
• Focuser: It is a tube consisting of an eyepiece that can be moved using knobs to adjust the focus of an image.
• Finderscope: It is present in the telescope tube to find out the direction of the object to be observed. 
• Star Diagonal: It helps to see through a telescope comfortably by adjusting the light coming from a telescope.
• Mount: Mount is used for holding a telescope and helps to rotate it in the direction of the object.
• Tripod: A tripod is where the mount and a telescope are placed. Tripod has height-adjustable legs that prevent the telescope from vibrating during the observation. 

Parts of Telescope

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Types of Telescopes

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Telescopes can be classified into two major categories which are:

1. Reflecting Telescopes: Telescopes using Mirrors.
2. Refracting Telescopes: Telescopes using Lenses.

There are some telescopes that use both lenses and mirrors.

Reflecting Telescopes

• Reflecting Telescopes are more powerful than refracting telescopes. 
• Reflecting Telescopes have a curved mirror at the bottom of the tube. 
• The curved mirror focuses the light at a point in the tube. 
• A second mirror is placed in the way of the focused light. 
• The second mirror sends the light out the side of the tube, through an eyepiece. 
• A lens is placed in the eyepiece that magnifies the image formed by the light.
• The Hubble Space Telescope, which orbits Earth on a spacecraft, is an example of a reflecting telescope.

Newton's Reflecting Telescope

Refracting Telescopes

• Refracting Telescope is a type of telescope that uses a lens as its objective to form an image.
• It is a tube with one or more lenses at each end. 
• Light from the distant object enters the far end of the tube. 
• The lenses at that end called the objective lenses, bend the light. 
• They focus it to a point near the other end of the tube that forms an image, or picture of the object, at that point. 
• The lens or lenses at this other end are called the eyepiece, magnifying the image.
• The different types of refracting telescopes are the Galilean Telescope, Keplerian Telescope, and Achromatic Refractors.

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Uses of Telescope

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Telescopes are an integral component of Astronomy. There are several important functions of a telescope which are: 

• Observing Celestial Bodies from Earth’s Surface: Telescopes are used for observing celestial bodies such as planets and stars.
• Accurate Data Collection: Hubble Space Telescope (HST) is the best-known optical telescope that collected data for the deep universe.
• Analyzing Image and Light: Telescopes provide graphical representations of distant objects.

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Magnifying Power of a Telescope

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Magnifying Power is a measure of the extent to which an object can be enlarged/magnified through a telescope.

The formula for the magnifying power of a telescope is

Where 

• m is the magnifying power.
• f_o is the focal length of the objective.
• f_e is the focal length of the eyepiece.

The magnifying power of a telescope can also be given through the formula: 

Here, d is the least distance of distinct vision.

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Things to Remember

• Telescope is an optical instrument used to observe distant objects by collecting electromagnetic radiation.
• Galileo Galilei was the first person to use a telescope for the purpose of astronomy.
• The two major types of telescopes are Reflecting Telescopes and Refracting Telescopes. 
• Reflecting telescopes use mirrors while refracting telescopes use lenses to magnify objects.
• Optical Tube, Eyepiece, Focuser, Finderscope, Star Diagonal, Mount, and Tripod are the parts of a telescope. 
• The magnifying power of a telescope is given by the formula m = f_o/f_e.

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Previous Year Questions

1. An astronomical telescope has objective and eyepiece of focal lengths… (NEET 2016)
2. In an astronomical telescope in normal adjustment a straight black… (NEET 2015) 
3. A telescope has an objective lens of 10 cm diameter and is situated…
4. An astronomical refracting telescope will have large angular… (NEET 2018)
5. The magnifying power of an astronomical telescope… (JIPMER 1995)
6. An astronomical telescope has an angular magnification of… (JEE Advanced 1989)
7. A telescope has an objective of focal length 100 cm… (BCECE 2009)
8. An astronomical telescope of tenfold angular magnification… (NEET 1997)
9. Calculate the limit of resolution of a telescope objective… (JEE Main 2019)
10. Diameter of the objective lens of a telescope is… (JEE Main 2019)