Maharashtra Board Class 9 Science Chapter 18 Observing Space Telescopes PDF Download

18. Observing Space: Telescopes

Can You Recall?

From early days, man has been curious about the sun as well as the moon and stars seen in the night sky. Using his boundless imagination, he tried to understand the sky as observed by the naked eyes. He noticed that the position of the stars changed with time and had something to do with the occurrence of seasons. As the knowledge of the cycle of seasons was necessary for agriculture, sky watching began to prove useful to him. The position of the constellations was also useful to sea goers for navigation. Man began to make determined efforts to find answers to questions which arose out of his sky watching. But he did not have any equipment to get a closer view of the stars and planets in the sky.

Today, 400 years after Galileo's use of the telescope, tremendous progress has been made in telescope technology and in space science and technology on the whole. This great leap in technology has helped to construct for us an astounding picture of our universe. Space science and technology are not only important for research purposes, but also to help provide us with many of the comforts and facilities we enjoy in our everyday life. A telescope is used to observe space, but will one telescope be sufficient for us to observe space completely? Why do we need different telescopes for the purpose? Are telescopes installed even in space? In this chapter, we are going to study the science behind many such questions.

An Introduction To Scientists

In 1608, spectacle maker and researcher, Hans Lippershey discovered that seeing through two lenses kept one behind the other, seems to bring objects closer to us. He thus made the first telescope. Galileo made a telescope in 1609 and used it for space observations. He realized that there are many more stars than what could be seen with naked eyes. Using his telescope, he also discovered the moons of Jupiter, the black spots on the sun, etc.

Teacher's Note

Just like how Galileo used his telescope 400 years ago, today we use bigger and better telescopes to see far away stars and planets in space. This shows how science helps us discover new things.

Exam Trick

Remember: Galileo saw Jupiter's moons in 1609. This is the first time someone used a telescope to see space. Always remember Galileo = first space observer.

Points to Remember

Hans Lippershey made the first telescope in 1608 using two lenses.
Galileo used a telescope in 1609 to look at space.
Galileo discovered moons of Jupiter and black spots on the sun.
A telescope helps us see distant stars and planets clearly.

Different Forms Of Light

Light is an electromagnetic wave. Every wave has a characteristic wavelength. Our eyes can see only that light which has wavelengths between 400 nm to 800 nm. Such light is called visible radiation. However, there are electromagnetic waves of wavelengths other than the visible ones most of which we cannot 'see' as our eyes are not sensitive to them.

Study the following table.

1 nm (nanometer) = 10^-9 m and 1 pm (picometer) = 10^-12 m

Of all the above types, our eyes are only capable of seeing the visible radiation. Thus, we use the visible radiation telescopes i.e. optical telescopes, made from regular lenses or mirrors to see the visible radiation coming from the space. However many heavenly bodies emit radiations other than the visible light. Thus we need different types of telescopes like the X-ray, gamma-ray and radio telescopes to receive such radiation and to study their sources.

Teacher's Note

Our eyes can only see visible light between 400 nm to 800 nm. Just like how we cannot hear sound above a certain frequency, our eyes cannot see light above or below this range. This is why we need special telescopes to see other types of radiation.

Exam Trick

Remember: Visible light = 400 nm to 800 nm. This is the only light our eyes can see. All other radiations are invisible to us without special telescopes.

Points to Remember

Light is an electromagnetic wave with a characteristic wavelength.
Visible light has wavelengths between 400 nm to 800 nm only.
Our eyes can only see visible radiation.
There are many other types of radiation like radio waves, X-rays, and gamma rays that we cannot see.
We need special telescopes to see these invisible radiations.

Telescopes

Optical Telescopes

Most optical telescopes are made with two or more lenses as shown in figure 18.1. To collect the maximum amount of light coming from a heavenly object, the objective lens should be made as large as possible. Using the light collected by the objective a smaller lens, called the eyepiece, produces a large image of the source. Light rays change their direction as they enter a lens from the atmosphere and again when they enter the atmosphere after passing through the lens. This is called refraction. Hence such telescopes are called refracting telescopes. We shall study image formation by lenses in the next standard.

Even though such a telescope is useful for space observations, it presents certain difficulties.

1. As we saw above, if we wish to obtain a bright image of a source by collecting the maximum possible light from it, the objective lens must be made as large as possible. However, it is very difficult to make very large lenses. Also, large lenses are very heavy and tend to get distorted.

2. As the objective and eyepiece are placed at the opposite ends of the telescope, the length of the telescope also increases with increase in the size of the lenses and the telescope becomes difficult to manage.

3. The images formed by lenses have errors of colours. This is called chromatic aberration.

Teacher's Note

An optical telescope uses lenses to collect and magnify light from distant stars. Think of it like using your hand as a funnel to collect more rainwater compared to just looking at the rain with your eyes.

Exam Trick

Remember: Optical telescope = uses lenses. The objective lens must be large to collect more light, but large lenses are heavy and difficult to make. This is the main problem with optical telescopes.

Points to Remember

Optical telescopes have an objective lens and an eyepiece lens.
The objective lens collects light from stars and planets.
The eyepiece produces a magnified image of the source.
Light bends when passing through lenses, which is called refraction.
Making very large lenses is difficult because they become heavy and distorted.

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