Solution-Class 9-Science-Chapter-18-Observing Space : Telescopes-Maharashtra Board

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

b. GMRT is used for radiowaves.

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

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

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

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.
• Besides, temperature and air pressure causes the atmospheric turbulence and thus, light rays changes their path and shakes 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.

This issue is overcome either of the following ways :

• To reduce the intensity of these problems, optical telescopes are situated on mountain top, away from inhabited places. However, limitation caused due to atmosphere still persist.
• To get rid of problems in situating telescope on the earth, scientists have successfully placed the telescope in the space.

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:
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.

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.

Telescope shown in the diagram is Newtonian type of telescope.

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

Another telescope which uses a concave mirror is Cassegrain type of optical telescope.

The telescope shown above has a concave mirror, a plane mirror and an eyepiece. This is a Newtonian type of telescope.

• The distance between concave mirror and plane mirror can be adjusted and thus, the focal length is also adjustable.
• The light rays coming in from the distant object are parallel. When these parallel rays fall on the concave surface of mirror, they reflect back as a diminished and inverted image on the plain mirror in front.
• This image is again reflected back and serves as an object for the eyepiece and acts like a simple microscope giving a magnified original image.
• Observer then can see this image through an eyepiece.

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. Both lenses are arranged in a such a way that the distance between them can be adjusted. The objective was a convex lens and eye piece was a concave lens.

These telescopes are used to see radio waves coming from space.

Many heavenly bodies like stars, planets, galaxies, emit radiations other than visible light like X-rays, gamma rays, infrared rays, etc. Radio Telescopes capable to receive and produce images from such rays.

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.

A giant radio telescope can be formed by linking mini radio telescopes together. Giant Wave Radio Telescope (GMRT) is erected at Narayangaon near Pune. This large radio telescope is actually an interconnected 30 parabolic dishes spread over 25 sq. km area.

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.

X-ray telescope cannot be based on the Earth because X-rays cannot reach up to surface of the earth due to certain constraints like atmosphere, temperature, air pressure, cloud cover, etc.