CHAPTER 3: Light and Telescopes

Transcription

1 CHAPTER 3: Light and Telescopes

2 WHAT DO YOU THINK? What is light? Which type of electromagnetic radiation is most dangerous to life? What is the main purpose of a telescope? Why do stars twinkle? What type(s) of electromagnetic radiation can telescopes currently detect?

3 You will discover the connection between sunlight, radio waves, and other kinds of electromagnetic radiation the debate over what light is, and how Einstein resolved it how telescopes collect and focus light what telescopes can and cannot do why different types of telescopes are used for different types of research what new generations of land-based and space-based high-technology telescopes being developed can do how astronomers use the entire spectrum of electromagnetic radiation to observe the stars and other astronomical events

4 Prisms Newton found that the prism itself does not add the colors to the light, but that color is an intrinsic property of light.

5 In 1860, James Clerk Maxwell combined and unified the current theories of electricity and magnetism and showed that electric and magnetic fields should travel through space together in the form of electromagnetic waves. These waves are characterized by their wavelength, the distance between two peaks in a wave.

6 There are a wide range of wavelengths of electromagnetic waves plotted on the electromagnetic spectrum. We classify these waves by their source, use, or interactions with other matter. Only a very small range of wavelengths, 400nm to 700nm, are visible to humans. (Since these wavelengths are small we describe them in terms of nanometers (10-9 m) or Angstroms (10-10 m). Other wavelengths are classified as Gamma Rays, X-Rays, Ultraviolet, Infrared, Microwaves or Radio waves.

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12 Ole Romer used two eclipses of one of Jupiter s moons to show that light does not travel infinitely fast and to measure its speed. One can also use Maxwell s equations to calculate the speed of light. The speed of light, denoted by the letter c, has since been measured to be 299, km/s which we generally round to c = 300,000 km/s = 180,000 mi/s

13 The Doppler Effect Sources moving toward the observer squeeze light waves in front of them, causing them to be shorter. We call this a blueshift. Sources moving away from the observer stretch the light waves behind them, causing them to be longer. We call this a redshift.

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15 Einstein showed that light also behaves as a particle. He found that light behaves as discreet packets called photons. The energy of a photon decreases with its wavelength and is calculated by: Photon energy = Planck s constant speed of light wavelength

16 The transparency of a material depends on the wavelength of light. Earth s atmosphere is relatively transparent to visible light and radio waves, which are referred to as windows through which we can view space from a ground-based telescope.

17 TWO BASIC TYPES OF TELESCOPES 1. Reflecting Telescopes use a mirror to gather and collect light. 2. Refracting Telescopes use a lens to gather and collect light.

18 REFLECTING TELESCOPES The reflected angle is the same as the incident angle. Light rays approaching the surface of a mirror will bounce off at the same angle at which it approaches. This is the principle of reflection.

19 The large mirror used to gather and focus the light in a reflecting telescope is called the primary mirror. The surface of the mirror used is bent into a curve. Parallel light rays from distant objects converges to a focal point. The distance between the mirror and its focal point is called the focal length.

20 Even though light rays from stars spread out in all directions, they must travel over huge interstellar distances to reach the Earth. Therefore, the rays which enter our telescopes are essentially traveling in the same direction, and thus they are considered parallel. If the object we are examining is an extended object, such as the Moon, then the light rays converge in a focal plane rather than a single point.

21 A Newtonian telescope uses a flat secondary mirror to redirect the focused image to the side of the telescope for viewing. The image is viewed through a small focal length lens called an eyepiece.

22 Designs of Reflecting Telescopes

23 FUNCTIONS OF A TELESCOPE Telescopes brighten objects by collecting light. The ability of a telescope to collect light is called its light-gathering power. SMALLER DIAMETER LESS LIGHT GATHERING POWER DIMMER IMAGE LARGER DIAMETER MORE LIGHT GATHERING POWER BRIGHTER IMAGE

24 FUNCTIONS OF A TELESCOPE Telescopes reveal the details of extended celestial objects. The clarity of the image and the amount of detail revealed is called the angular resolution. Smaller diameter telescopes have less resolution and produce images that are blurred. Larger diameter telescopes have more resolution and produce images that are clear.

25 FUNCTIONS OF A TELESCOPE Telescopes make objects appear larger. This is called magnification. The magnification is the focal length of the primary mirror divided by the focal length of the eyepiece. magnification = focal length of the primary focal length of the eyepiece A common misconception is that the magnification is the most important factor determining the quality of a telescope s images. Actually, the light-gathering and resolving powers of a telescope are much more important, as the magnification can be changed simply by switching eyepieces. Telescopes have a maximum useful magnification beyond which images will be larger but will no longer be clearly focused. In general, the larger the diameter of a telescope, the greater the maximum magnification will be.

26 New technology has created chargecoupled devices (CCDs) to gather light more efficiently than photographic plates. These devices produce images with astounding detail. Shown in this picture are the intricate details of the Rosetta Nebula 5000ly away.

27 Ordinary Photography Versus CCDs ORDINARY PHOTOGRAPH NEGATIVE NEGATIVE USING CCDs COMBINED CCD IMAGES USING COLORED FILTERS

28 REFRACTING TELESCOPES Light rays traveling into a transparent medium such as glass bend at the surface. The bending of light rays between two transparent media is called refraction. If the lens is curved, parallel rays will converge at a focal point, just like the rays in a reflecting telescope.

29 Like reflecting telescopes, extended objects produce images in a focal plane. Refracting telescopes use an objective lens to gather light and an eyepiece through which the image is viewed.

30 Limitations of Refracting Telescopes Different colors of light are refracted differently and have different focal points. Thus, all the colors of the image will not be focused at once. This is called chromatic abberation. It is difficult to grind a lens into the proper shape to have all parallel rays converge at a single focal point. The weight of a large lens can cause the lens to sag and distort the image. Air bubbles in the glass cause unwanted refractions, distorting the image. Glass is opaque to certain wavelengths of light, meaning they do not go through the glass. THE LARGEST REFRACTING TELESCOPE AT YERKES OBSERVATORY

31 These issues do not affect reflecting telescopes because the light from the stars does not travel through a glass lens before being focused. However, reflecting telescopes do have some problems. One problem is that the secondary mirror used to deflect the light out the side partially blocks the light from the star.

32 Another problem with reflecting telescopes is called spherical aberration. When a sphericallyshaped mirror is used, the light rays hitting far from the center will not converge at the same point. One solution is to grind the mirror into a parabolic shape. Another solution is to use a correcting lens to make all the light rays converge at a single point.

33 Making a Large Parabolic Mirror 40,000 pounds of glass are loaded into a rotated furnace and heated to 1500K. After melting, spinning and cooling, the surface is ready to be coated with a highly reflective material.

34 Ground Based Telescopes are Limited Our atmosphere causes the stars to appear to shift in color and brightness. This causes the star to appear to flicker. We call this twinkling. GROUND BASED TELESCOPES BLURRED BY TWINKLING TELESCOPES IN ORBIT ABOVE OUR ATMOSPHERE AVOID THIS PROBLEM

35 Light from cities scatters in our atmosphere reducing the visibility of celestial objects. This is called light pollution, and has been an increasing problem in recent years. The view from Kitt Peak National Observatory of the Tuscon, Arizona skyline in 1959 The same skyline in 1972

36 To avoid adverse effects of our atmosphere, we launch telescopes on satellites that circle the Earth above the atmosphere. THE HUBBLE SPACE TELESCOPE (HST)

37 Using adaptive optics to calculate the amount of twinkling of our atmosphere, as well as changes to the shape of the mirror, we can receive better images from ground based telescopes. IMAGES OF SATURN GROUND BASED WITH NO ADAPTIVE OPTICS GROUND BASED WITH ADAPTIVE OPTICS HUBBLE IMAGE

38 NONOPTICAL ASTRONOMY Using interferometry to combine the information from two or more telescopes to create one image, radio telescopes can be linked together over large distances to overcome the limits of resolution for a single radio telescope. The radio telescope in Greenbank, West Virginia, has an off-center secondary which does not block incoming radio waves. The Very Large Array (VLA) in central New Mexico combines the signals from 27 radio telescopes to either cover wide areas of the sky or increase the resolution for a small area.

39 NONOPTICAL ASTRONOMY The Murchison Widefield Array (MWA) is a low-frequency radio telescope operating between 80 and 300 MHz. It is located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia.

40 Images of Saturn IN VISIBLE LIGHT IN RADIO WAVES NOTE: We use falsecolor images when displaying data from non-visible light.

41 The Spitzer Space Telescope will examine infrared cosmos Mirror Setup Launched in 2003

42 Non Optical Images of the Sun Reveal Details which Cannot be Observed in Visible Light X-RAY VISIBLE

43 Different Views of Our Universe We are familiar with the view of our surrounding universe in visible light

44 Astronomers also study our universe using other wavelengths of light to unlock its secrets RADIO WAVES INFRARED X-RAYS GAMMA RAYS

45 WHAT DID YOU THINK? What is light? One form of electromagnetic radiation with both wave and particle properties. Which type of electromagnetic radiation is most dangerous to life? Gamma rays. What is the main purpose of a telescope? To collect as much light as possible. Why do stars twinkle? Rapid changes in the density of the Earth s atmosphere cause passing light starlight to change direction. What type(s) of electromagnetic radiation can telescopes currently detect? Some can detect the entire electromagnetic spectrum.

46 Key Terms active optics adaptive optics angular resolution (resolution) blocked light blueshift Cassegrain focus charge-coupled device (CCD) coude focus Doppler shift electromagnetic radiation electromagnetic spectrum eyepiece lens focal length focal plane focal point gamma ray infrared radiation interferometry light-gathering power magnification Newtonian reflector objective lens photon pixel primary mirror prime focus radio telescope radio wave redshift reflecting telescope reflection refracting telescope refraction Schmidt corrector plate secondary mirror seeing disk spectrum spherical aberration twinkling ultraviolet (UV) radiation very-long-baseline interferometry (VLBI) wavelength X ray