Chapter 18 - Observing Space : Telescopes
1. Fill in the blanks with the proper words.
1. The wavelength of visible light is between …….. and ………
Ans: 400 nm, 800 nm
2. GMRT is used for ……. waves.
Ans: radio
3. A certain X-ray telescope is named after scientist ………..
Ans: Subramanian Chandrashekhar
4. The first scientist to use a telescope for space observation was ………………. .
Ans: Galileo Galilei
5. The biggest optical telescope in India is situated at ……….
Ans: Aryabhatta Research Institute of Experimental Science, Nainital
2. Form pairs
| ‘A’ Group | ‘B’ Group |
|---|---|
(i) X-rays | (a) GMRT |
(ii) Optical Telescope | (b) ISRO |
(iii) Indian radio telescope | (c) Hubble |
(iv) Launching artificial satellites | (d) Chandra |
Ans:
| ‘A’ Group | ‘B’ Group |
|---|---|
(i) X-rays | (d) Chandra |
(ii) Optical Telescope | (c) Hubble |
(iii) Indian radio telescope | (a) GMRT |
(iv) Launching artificial satellites | (b) ISRO |
3. What are the difficulties in using ground based optical telescopes? How are they overcome?
Ans:
Difficulties:
(i) The visible light coming from a heavenly body has to pass through the earth’s atmosphere to reach the earth’s surface.
(ii) During this journey, some of the light is absorbed by the atmosphere, and the intensity of the light reaching the earth’s surface decreases.
(iii) A second problem is caused by changes in atmospheric pressure and temperature.
(iv) These changes cause turbulence in the atmosphere, which in turn causes the light rays to change their path slightly and thereby shake the position of the image.
(v) We cannot use an optical telescope during the day because of sunlight.
(vi) During the night, too, city lights and cloudy weather can cause difficulties in observing the heavenly bodies.
Solution:
(i) To reduce these problems, optical telescopes are situated on top of mountains in uninhabited places.
(ii) To get rid of all the above problems completely, the telescope is placed above the earth’s atmosphere, in space.
4. Which type of telescopes can be made using a concave mirror, convex mirror, plane mirror and a lens? Draw diagrams of these telescopes.
Ans:
A Newtonian telescope can be made using a concave mirror, plane mirror and a lens.
A Cassegrain telescope can be made using a convex mirror, concave mirror and a lens.
A refracting telescope can be made using two or more lenses.
5. Study the figure and answer the following questions
a. What type of telescope is shown in the figure?
Ans: The Newtonian Telescope (Reflecting Telescope)
b. Label the main parts of the telescope.
Ans:
(1) Concave Mirror
(2) Eye Piece
(3) Light Source
(4) Plane Mirror
(5) Light Source
c. Which type of mirror does the telescope use?
Ans: Concave mirror
d. What other type of telescope uses a curved mirror?
Ans: The Cassegrain telescope
e. Explain the working of the above telescope.
Ans:
(i) Light rays coming from space are reflected by the concave mirror.
(ii) Before these reflected rays converge at the focus, they are deflected again by a small plane mirror.
(iii) As a result, they get focused at a point lying on the perpendicular to the axis of the telescope’s cylinder.
(iv) They pass through the eyepiece, and we get a magnified image of the source.
6. Answer the following questions.
a. Explain the construction of Galileo’s telescope.
Ans:
(i) The Galileo’s telescope is made up of two or more convex lenses.
(ii) The lens facing the object is called the objective lens which is made as large as possible to collect the maximum amount of light coming from a heavenly object.
(iii) The lens closer to the eye is called the eyepiece which uses the light collected by the objective lens and produces a large image of the source.
(iv) Such a telescope is also called a refracting telescope.
b. Explain the construction of a radio telescope.
Ans:
(i) The radio telescope is made from one or more dishes of a particular parabolic shape.
(ii) As in an optical telescope, the incident radio waves are reflected by these dishes and converge at the focus.
(iii) A radio receiver is placed at the focal point.
(iv) The information gathered by this receiver is passed on to a computer, which analyses it and constructs an image of the source.
c. Why are optical telescopes located in uninhabited places on mountains?
Ans:
(i) The visible light coming from a heavenly body has to pass through the earth’s atmosphere to reach the earth’s surface.
(ii) During this journey, some of the light is absorbed by the atmosphere, and the intensity of the light reaching the earth’s surface decreases.
(iii) A second problem is caused by changes in atmospheric pressure and temperature.
(iv) These changes cause turbulence in the atmosphere, which in turn causes the light rays to change their path slightly and thereby shake the position of the image.
(v) We cannot use an optical telescope during the day because of sunlight.
(vi) During the night, too, city lights and cloudy weather can cause difficulties in observing the heavenly bodies.
(vii) Hence, optical telescopes are located in uninhabited places on mountains.
d. Why can an X-ray telescope not be based on the earth?
Ans:
(i) X-rays are emitted from distant heavenly bodies.
(ii) The rays coming towards the earth will be absorbed by the earth’s atmosphere and go undetected.
(iii) Hence, X-ray telescopes are placed in an orbit outside the earth’s atmosphere and not on the earth.