1446 Introductory Astronomy II

1. 1446 Introductory Astronomy II Chapter 3 Light, EM Radiation & Telescopes R. S. Rubins Fall 2011

2. Dispersion of White Light by a Prism • The upper picture shows the variation of wavelength with color. • The lower picture shows Newton’s experimental proof that the glass changes the direction of a light-beam, but does not affect its color.

3. Dispersion of Light • By passing white light through a prism, Isaac Newton (ca. 1660) showed white light to be composed of all the colors of the rainbow: red, orange, yellow, green, blue, violet. • Dispersion is the process of separating light into its constituent colors. • The passage of light is a transfer of energy, which was known to occur either through the agency of particles or of waves. • Newton considered light to be a stream of particles, while the Dutch scientist, Huygens, proposed it to be a wave motion. • Over 100 years later, Young (1801) used the phenomenon of interferenceto show experimentally that light travels as waves,

4. The Visible Spectrum

5. The Speed of Light 1 • The Danish astronomer, Ole Romer (ca. 1675) used Newton’s Laws to predict the eclipses of the moons of Jupiter, and found them to be early when Jupiter was near conjunction, and late when near opposition. • He deduced that light must travel at a finite speed. His value c ≈ 230,000 km/s was low, because a precise Sun-Jupiter was not known at that time. • The time t that it takes for light to travel a distance x in space is obtained from the equation x = ct, where c = 300,000 km/s is the speed of light in a vacuum. • Thus, the time it takes for light from the Moon to reach us is t = x/c = 384,400 km/ 300,000 km/s ≈ 1.3 s.

6. The Speed of Light 2

7. The Speed of Light 3 The rotation rate is adjusted, so that, in the time taken for light to travel there and back to Mt. Wilson, one face of the mirror is replaced by the next. Speed of light = distance/time = 2d/(T/8) = 16d/T T is the time for one complete revolution of the mirror, and d = 35 km.

8. Why Study Electromagnetic Waves? • To study any object in the universe beyond our local area of the Solar System requires that we use electromagnetic (EM) radiation, most often visible light for very distant objects). • As long as the object emits an EM signal, which can be observed in a telescope, the signal can be analyzed, and will give information about the types of atoms it contains, its temperature, how fast it is moving towards or away from us, and usually, how far it was from us when it emitted the signal reaching us. • Since any EM signal travels at the speed of light, we can find out how long ago the signal was emitted; i.e. we are able to look back in time.

9. Some Definitions • A light ray is a very narrow beam of light. • The deviation is the angle by which a light ray is deflected from in its initial direction of travel, by (for example) a prism, a rectangular block, or a diffraction grating. • Reflection is the rebound of a light ray off a surface. • Refraction is the bending of a light ray when passing from one transparent medium to another at an oblique angle. • The denser the medium, the slower the speed of light. • In a medium of refractive index n, the speed of light is v = c/n.

10. Reflection of Light The angle of incidence i equals the angle of reflection r.

11. Refraction of Light

12. Wave Properties of Light • A monochromatic beam contains light of a single wavelength. • Diffraction is the bending of light behind an aperture or around an obstacle. • Interference is the adding of “displacements” which occurs when two or more waves of the same type and wavelength meet at a point in space. • Polarization occurs when the vibration of a (transverse) wave occurs in one direction only. • All waves obey the equation, v = f  , where v is the speed of the wave in km/s, f is the frequency in Hz (hertz = cycles/sec), and  is the wavelength in km.

13. A Continuous Wave • The wavelength  is the distance between two neighboring points with the same phase, such as two neighboring crests. • For a monochromaticwave moving with speed v, the number of crests passing any point per second is the frequencyf. • The speed, v = f λ, depends only on the type of wave.

14. Rainbow This is an example of refraction, reflection and dispersion.

15. Interference of Two Like Waves 1 Constructive interference Destructive interference

16. Interference of Two Like Waves 2 Constructive Interference Destructive Interference

17. Diffraction of Light Diffractionis the bending of light behind apertures and around obstacles. Light passing through a very narrow circular aperture (Fig. c), behaves as though it originated from a point.

18. Double Slit Interference 1 • Monochromatic light from a single source is passed through two narrow slits separated by a distance of the order of λ. The separation of the slits is d.

19. Double Slit Interference 2 Light diffracted through slit S0 reaches slits S1 and S2. Interference of the light passing through the two slits gives a pattern of light and dark lines on the screen, termed interference fringes. 19

20. Double Slit Interference 3smaller spacing larger spacing

21. Diffraction Grating • A diffraction grating, which consists of thousands of equally spaced fine lines ruled on a small rectangular plastic slide, makes the interference lines very sharp. Two slits Six slits Every type of atom, ion or molecule is distinguished by its set of spectral lines.

22. A Hypothetical Line-Spectrum The prism shown above is used to represent a diffraction grating. 22

23. Line Spectra of some Elements 23

24. Grating Spectrometer collimator grating telescope • As the angle θ is increased, the m =1 line of wavelength λ is observed when d sinθ = λ, where d is the separation between the slits on the grating. eye

25. Dispersion by a Diffraction Grating The m=0 spectrum shows as a single line, which appear white for a rainbow of colors. The diffracted spectra are denoted m =1 and m = 2, and the short wave (blue) end of each spectrum is closer to the m = 0 line than is the (long wave) red end. Emission line-spectrum Continuous spectrum 25

26. The Electromagnetic Spectrum 3

27. The Electromagnetic Spectrum 2

28. An Electromagnetic Wave • An EM wave has both electric and magnetic fields. • The wave in the figure is said to be polarized, since each field vibrates in only one direction. • Most radiation observed from space is unpolarized, i.e. the electric and magnetic fields vibrate in all directions.

29. Photons • All EM radiation travels through space at the speed c. • While EM waves travel through space as waves, their interaction with matter is as tiny packets of energy, known as photons (Einstein, 1905). • The energy E of a photon is given by E = hf = hc/λ, where h is Planck’s constant. • In practical units, the energy of a photon is given by E = 1240/λ, • where E is in eV (electron-volts) and λ is in nm. • Photons longer λ of have lower energies, and vice-versa. • Example: a photon of wavelength 310 nm has energy E = 1240/310 = 4.0 eV

30. The Quantum World: Waves and Particles 1 • The disagreement between light as waves (Huygens) and light as particles (Newton) was apparently resolved over a century later by Young in 1801, who showed through an interference experiment, that light was a wave-like phenomenon. • About 70 years later, when the photoelectric effect was studied, the wave model for light completely failed to explain the observational details. • However, using a simple particle model of light, in which each light particle (photon) carried an energy hc/λ, Einstein (1905) was able to quantitatively explain all aspects of the photoelectric effect.

31. The Quantum World: Waves and Particles 2 • If one accepts that an EM wave also has particle-like behavior, then one might expect the reverse to be true. • In 1923, de Broglie showed that the allowed orbits of Bohr’s theory of the H atom (see Chapter 4) could be explained by electron wavelengths λ = h/mv, where v is the speed of the electron in its orbit. • Quantum theory, originally formulated in 1925, provided a mathematical formalism for explaining the wave and particle properties of fundamental particles. • The wave nature of particles was verified directly by the observation of electron diffraction in 1927, and later by neutron diffraction.

32. The Quantum World: Waves and Particles 3 • Although the standard interpretation of quantum theory was criticized by a number of famous physicists, including Einstein, the remainder of the twentieth century has been a period of triumph for the theory, in which many curious effects were explained, and strange predictions verified. • There remains one major problem, which is the distinct separateness of quantum theory and general relativity, both of which have been remarkably successful. • The integration of these two theories into what might be called “quantum gravity” is a project for this century. • Finally, we should remember that the universe behaves as our observations guide us, and if we have no picture for a particle that is also a wave, there is nothing that can be done about it.

33. Transparency of Earth’s Atmosphere IR rays reach high mountains, but gamma rays, X rays and the “far” UV are strongly absorbed by the atmosphere.

34. Radio Telescopes 1 The Green Bank Telescope (GBT) in West Virginia has a dish about 100 m in diameter, and is the world’s largest rotatable radio telescope.

35. Other Types of Telescope • Water vapor is the main absorber of infrared radiation from space, so that surface IR telescopes must be placed in exceptionally dry locations, such as at the Summit of Mauna Kea in Hawaii. • However, the best locations for IR , UV , X-ray and gamma-ray telescopes are on orbiting satellites. • Normal methods of reflection do not work for X rays and gamma rays. • X-rays, which can be reflected when grazing a surface, are focused with a “grazing incidence” X-ray telescope. • High energy particle-physics techniques are used in building gamma ray instruments.

36. Resolving Power 1 • The resolving powerof an optical instrument is its ability to distinguish between closely spaced objects. • The headlights are resolved in Fig.(a), only just resolved in Fig.(b), and unresolved in Fig.(c) • Lenses (including the eye) and telescopes give images of point-objects which are diffraction patterns.

37. Resolving Power 2 • The resolvingpower (RP)of a lens is defined as the distance S between two points, which can just be resolved in the lens image. • When two images are just resolved, the central maximum of one image coincides with the first minimum of the other. • Increasing the magnification of a telescope without changing the RP, increases the size of the diffraction pattern, but not the detail observed. Central maximum First minimum

38. Resolving Power 3 • High resolution in a telescope requires an objective lens (or mirror) of large diameter, and radiation of short wavelength. • A large diameter lens/mirror also has the advantage of increasing the light-gathering ability of the telescope, thus allowing fainter objects to be observed. • To obtain the resolution of a single gigantic telescope, arrays of small telescopes are linked together electronically in a process known as interferometry. • This process gives the resolution of a large telescope, but not its light-gathering ability. • TheVery Large Array(VLA) in New Mexico has 27 dishes, with a resolution equivalent to a single 40 km diameter mirror, while the Very Long Baseline Array(VLBA) extends from the Caribbean to Hawaii.

39. Radio Telescopes 2 • Each of the Keck Reflecting Telescopes in Hawaii is a 10 m reflector. • When linked together using interferometry,they are equivalent to a single 85 m telescope.

40. Radio Telescopes: the Very Large Array 1

41. Radio Telescopes: the Very Large Array 2

42. Radio Signal Compared • The photos compare a spacecraft camera photo of Saturn with a radio image from the VLA, showing the low resolution of the latter. • To be displayed as a photo, the radio signal must be shown in “false” color. • The most intense radio emission is red, followed by yellow.

43. X-ray Telescope • The arrangement of nested cylindrical mirrors allows the X rays to be focused by reflections at grazing incidence.

44. Orion in Visible and IR Unlike visible light, IR is emitted by warm objects, so that IR may be used to study gas clouds. Also, huge dust clouds, which block visible radiation, allow passage of longer IR waves, enabling objects behind the dust clouds to be observed.

45. Orbiting Infrared Detectors Since IR measures the warmth of objects, it is necessary to cool the telescopes and detectors to liquid helium temperatures (below 4 K) in order to reduce interference from nearby objects. The first orbiting IR telescope, the Spitzer Space Telescope, launched in 2003, is nearing the end of its useful life, as its refrigerant evaporates, since there is no way of replacing the refrigerant. SOFIA (Stratospheric Observatory for Infrared Astronomy), which is contained in a modified Boeing 747, made its first post-modification flight in 2007. Since it returns to the ground after each flight, the refrigerant can be filled up before each flight.

46. SOFIA 2009 SOFIA is an IR observatory placed on a modified Boeing 747. It will fly at more than 40,000 ft, which is above 99% of the water vapor in the atmosphere. It should make over a 100 flights a year for the next 20 years. Telescope

47. A Sofia Investigation Infrared telescopes are ideally suited for studying protostars, which are not hot enough to emit much visible light.