Standard 9 Science and Technology - 18. Observing Space : Telescopes

Question 1:

Fill in the blanks with the proper words.

a. The wavelength of visible light is between ........ and ........ .

b. GMRT is used for ....... waves.

c. A certain X-ray telescope is named after scientist .......... .

d. The first scientist to use a telescope for space observation was ................... .

e. The biggest optical telescope in India is situated at ......... .

Answer 1:

a. The wavelength of visible light is between 400 nm and 800 nm.

b. GMRT is used for radio waves.

c. A certain X-ray telescope is named after scientist Subrahmanyan Chandrasekhar.

d. The first scientist to use a telescope for space observation was Galileo Galilei.

e. The biggest optical telescope in India is situated at Aryabhatta Research Institute of Experimental Sciences, Nainital.

Question 2:

Form pairs

‘A’ Groups ‘B’ Groups

(i) X-rays (a) GMRT

(ii) Optical Telescope (b) ISRO

(iii) Indian radio (c) Hubble telescope

(iv) Launching (d) Chandra

artificial satellites

Answer 2:

‘A’ Groups	‘B’ Groups
(i) X-rays	(d) Chandra
(ii) Optical telescope	(c) Hubble telescope
(iii) Indian radio	(a) GMRT
(iv) Launching artificial satellites	(b) ISRO

Question 3:

What are the difficulties in using ground based optical telescopes? How are they overcome?

Answer 3:

The difficulties faced in using ground based telescopes due to which it does not make good quality observations are:

Intensity of light rays reaching the Earth's surface decreases as some of the light is absorbed by the atmosphere.
The change in atmospheric pressure and temperature cause the light rays to change their path slightly and thus shake the position of the image.
We cannot use optical telescopes during day time because of the presence of sunlight.
Even in night, city lights and cloudy weather can cause hindrance in observing the heavenly bodies.

To overcome the above problems, telescopes are now being erected in space itself. Some of the telescopes in space are:

Hubble telescope: It is an optical telescope launched by the National Aeronautics and Space Administration in 1990. It is orbiting the Earth at a height of 589 km from it and has a mirror of diameter 94 inches.
Chandra: It is an X-ray telescope launched by the National Aeronautics and Space Administration in 1999. Special mirrors which can reflect X-rays are used in this telescope.

Question 4:

Which type of telescopes can be made using a concave mirror, convex mirror, plane mirror and a lens? Draw diagrams of these telescopes.

Answer 4:

Reflecting types of telescopes can be made using a concave mirror, convex mirror, plane mirror and a lens. Reflecting type of telescopes is mainly of two types:
(a) Newtonian telescope
Light rays coming from a heavenly body (distant object) get reflected by the concave mirror. Before the reflected rays converge at the focus of the concave mirror, it is again reflected by the plane mirror as shown. As a result, the rays pass through the eyepiece and we get a magnified image of the body.

(b) Cassegrain telescope
In this telescope, the light rays after reflection from the concave mirror, are reflected back towards it by a small convex mirror. Then the rays pass through the eyepiece placed at the back of the concave mirror as shown. The eyepiece produces a magnified image of the source.

Question 5:

Study the figure and answer the following questions

a. What type of telescope is shown in the figure?

b. Label the main parts of the telescope.

c. Which type of mirror does the telescope use?

d. What other type of telescope uses a curved mirror?

e. Explain the working of the above telescope.

Answer 5:

a. Newtonian telescope is shown in the given figure.

b.

c. Both plane and spherical mirror is used in this telescope. The spherical mirror is a concave mirror.

d. Cassegrain telescope also uses a curved mirror.

e. Light rays coming from a heavenly body (distant object) get reflected by the concave mirror. Before the reflected rays converge at the focus of the concave mirror, it is again reflected by the plane mirror as shown. As a result, the rays pass through the eyepiece and we get a magnified image of the body.

Question 6:

Answer the following questions.

a. Explain the construction of Galileo’s telescope.

b. Explain the construction of a radio telescope.

c. Why are optical telescopes located in uninhabited places on mountains?

d. Why can an X-ray telescope not be based on the earth?

Answer 6:

a. In 1906, Galileo constructed a telescope which provides an erect image of an object only with the help of two lenses. The Galileo's telescope consists of two lenses located at the ends of a long tube as shown in the figure below. The objective was a convex lens and eye piece was a concave lens.

b. These telescopes are used to see radio waves coming from space. It has one or more dishes of parabolic shape. The rays coming from the distant source are first reflected by these dishes which converges at the focus. A radio receiver is placed at the focal point, which gathers the information. The gathered information is then passed on to a computer which analyses it and construct an image of the source.

c. Optical telescopes situated on the surface of Earth, does not produce images of good quality of the following reasons:

Intensity of light rays reaching the Earth's surface decreases as some of the light is absorbed by the atmosphere.
The change in atmospheric pressure and temperature cause the light rays to change their path slightly and thus shake the position of the image.
We cannot use optical telescopes during day time because of the presence of sunlight.
Even in night, city lights and cloudy weather can cause hindrance in observing the heavenly bodies.

Thus, optical telescopes are located in uninhabited places and on higher altitudes such as on top of mountains to reduce the above problems in appreciable amount.

d. X-ray telescope cannot be based on the Earth because X-rays do not reach the Earth surface as they are blocked by the Earth's atmosphere.