Exercise Solutions (Based on Textbook Screenshots)

1. Fill in the blanks with the proper words.

1. The wavelength of visible light is between 400 nm and 700 nm.
   Note: These are approximate values; some sources might cite slightly different ranges like 380 nm to 750 nm.
2. GMRT is used for radio waves.
3. A certain X-ray telescope is named after scientist Chandra (Subrahmanyan Chandrasekhar).
4. The first scientist to use a telescope for space observation was Galileo Galilei.
5. The biggest optical telescope in India is situated at Devasthal, Nainital (ARIES - Aryabhatta Research Institute of Observational Sciences).
   Note: This refers to the 3.6m Devasthal Optical Telescope (DOT).

2. Form pairs:

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

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

5. Study the figure and answer the following questions.

(Assuming the figure shows a Newtonian reflecting telescope as is common for such questions)

1. What type of telescope is shown in the figure?
   It is a Newtonian reflecting telescope.
2. Label the main parts of the telescope.
   The main parts are:

   • Telescope Tube (Optical Tube Assembly)
   • Objective Mirror (Primary Mirror): A large concave mirror at the bottom/back end of the tube.
   • Secondary Mirror (Diagonal Mirror): A small, flat plane mirror placed at a 45° angle near the top/front end.
   • Eyepiece: Located on the side of the tube, where the observer looks.
   • Focuser: Mechanism to adjust the eyepiece for a sharp image.
3. Which type of mirror does the telescope use?
   It uses a concave mirror as the primary (objective) mirror and a plane mirror as the secondary (diagonal) mirror.
4. What other type of telescope uses a curved mirror?
   Other types of telescopes that use curved mirrors include:

   • Cassegrain telescope (concave primary, convex secondary)
   • Schmidt-Cassegrain telescope
   • Maksutov-Cassegrain telescope
   • Ritchey-Chrétien telescope (used in Hubble)

   Basically, all reflecting telescopes use curved mirrors.
5. Explain the working of the above telescope.
   In a Newtonian reflecting telescope:

   1. Light rays from a distant object enter the telescope tube.
   2. These parallel rays strike the large concave primary mirror at the back of the tube.
   3. The primary mirror reflects and converges the light towards a focal point.
   4. Before the light reaches this focal point, it is intercepted by a smaller, flat secondary mirror (diagonal mirror) positioned at a 45-degree angle.
   5. The secondary mirror reflects the light sideways, out of the main tube.
   6. This light then passes through an eyepiece, which magnifies the image, allowing the observer to see a larger, brighter view of the distant object.

6. Answer the following questions.

1. Explain the construction of Galileo's telescope.
   Galileo's telescope, also known as a Galilean telescope, is a type of refracting telescope.
   Construction:

   • Objective Lens: It uses a plano-convex or bi-convex lens as the objective (the lens facing the object). This lens has a relatively long focal length.
   • Eyepiece Lens: It uses a plano-concave or bi-concave lens as the eyepiece (the lens the observer looks through). This lens has a short focal length.
   • Tube: The two lenses are mounted at opposite ends of a tube, with the distance between them being approximately the difference between their focal lengths ($F_o - F_e$).

   Working: Light from a distant object passes through the convex objective lens, which converges the rays. Before these rays can form a real image, they are intercepted by the concave eyepiece lens. The eyepiece lens diverges the rays, making them appear to come from a magnified, virtual, and upright image.
   Characteristics: It produces an upright image, which is advantageous for terrestrial viewing, but it has a narrow field of view and suffers from aberrations.
2. Explain the construction of a radio telescope.
   A radio telescope is designed to detect radio waves from astronomical sources.
   Construction:

   • Antenna (Dish): The most prominent part is usually a large parabolic dish (or an array of dishes). This dish acts like the objective mirror in an optical reflecting telescope. Its surface is made of metal or wire mesh that reflects radio waves.
   • Feedhorn/Receiver: Located at the focal point of the dish, the feedhorn collects the reflected radio waves. It is connected to a sensitive radio receiver.
   • Amplifier: The weak radio signals collected are amplified significantly.
   • Signal Processor/Computer: The amplified signals are then processed by computers. This can involve filtering, integrating signals over time, and, in the case of arrays, combining signals from multiple dishes (interferometry) to achieve higher resolution.
   • Output/Display: The processed data can be used to create radio maps (images) of the sky, intensity plots, or spectra of the radio source.

   Large radio telescopes like the GMRT in India consist of many individual dishes spread over a large area, working together as an interferometer.
3. Why are optical telescopes located in uninhabited places on mountains?
   Optical telescopes are located in uninhabited places on mountains for several key reasons:

   • Reduced Light Pollution: Uninhabited areas are far from city lights, which cause light pollution. Light pollution brightens the night sky and makes it difficult to observe faint astronomical objects.
   • Thinner Atmosphere: At higher altitudes on mountains, the layer of Earth's atmosphere above the telescope is thinner. This reduces:
     • Atmospheric Distortion (Seeing): Less air turbulence means sharper images (better "seeing").
     • Atmospheric Absorption: Less absorption of starlight by atmospheric gases.
   • Clearer Skies: Mountain locations are often above cloud layers and have less dust and moisture in the air, leading to more clear nights for observation.
   • Darker Skies: The combination of remoteness and altitude contributes to darker skies, essential for detecting faint objects.
4. Why can an X-ray telescope not be based on the earth?
   An X-ray telescope cannot be effectively based on Earth because Earth's atmosphere absorbs almost all X-rays coming from space. The atmosphere acts as a protective shield, preventing harmful high-energy radiation like X-rays and gamma rays from reaching the surface. While this is crucial for life on Earth, it means that X-ray astronomy can only be conducted from space, using telescopes placed on satellites orbiting well above the atmosphere (e.g., Chandra X-ray Observatory, Astrosat).