1. Slide 1 Light and telescopes Just by analyzing the light received from a star, astronomers can retrieve information about a star’s 1.Total energy output 2.Surface temperature 3.Radius 4.Chemical composition 5.Velocity relative to Earth 6.Rotation period

2. Slide 2 What is light?

3. Slide 3 Electricity

4. Slide 4 Magnetism

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6. Slide 6 Electromagnetic induction Time-dependent magnetic field creates time-dependent electric field, and vice versa

7. Slide 7 Electromagnetic waves

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9. Slide 9 Light as a Wave (1) Light waves are characterized by a wavelength  and a frequency f. f = c/ c = 300,000 km/s = 3*10 8 m/s f and are related through

10. Slide 10 Wavelengths and Colors Different colors of visible light correspond to different wavelengths.

11. Slide 11 The Electromagnetic Spectrum Need satellites to observe Wavelength Frequency High flying air planes or satellites

12. Slide 12 Light as a Wave (2) Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å): 1 nm = 10 -9 m 1 Å = 10 -10 m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).

13. Slide 13 Light as Particles Light can also appear as particles, called photons (explains, e.g., photoelectric effect). A photon has a specific energy E, proportional to the frequency f: E = h*f h = 6.626x10 -34 J*s is the Planck constant. The energy of a photon does not depend on the intensity of the light!!!

14. Slide 14 Dual, wave-particle nature of light 1 eV = 1.6x10 -19 J c = 3x10 8 m/s 1 Angstrom = 10 -10 m Speed of light in matter: c m = c/n, where n is refractive index Note: n is a function of

15. Slide 15 Stars are hopelessly far away … Matter in space consists of the same atoms as matter on Earth Physical laws should be the same We can still learn something about the stars!

16. Slide 16 Optical Telescopes Astronomers use telescopes to gather more light from astronomical objects. The larger the telescope, the more light it gathers.

17. Slide 17 Refractors and Reflectors (SLIDESHOW MODE ONLY)

18. Slide 18 Refracting/Reflecting Telescopes Refracting Telescope: Lens focuses light onto the focal plane Reflecting Telescope: Concave Mirror focuses light onto the focal plane Almost all modern telescopes are reflecting telescopes. Focal length

19. Slide 19 Disadvantages of Refracting Telescopes Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect). Can be corrected, but not eliminated by second lens out of different material. Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges

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21. Slide 21 140-ft Hevelius telescope 1673

22. Slide 22 Newton’s telescope: the first reflecting telescope

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25. Slide 25 Telescope parameters Light-gathering power (ability to see faint objects) Resolving power (ability to see fine details) Magnification (least important)

26. Slide 26 The Powers of a Telescope: Size Does Matter 1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared: A =  (D/2) 2 D

27. Slide 27 The Powers of a Telescope (2) 2. Resolving power: Wave nature of light => The telescope aperture produces fringe rings that set a limit to the resolution of the telescope.  min = 1.22 ( /D) Resolving power = minimum angular distance  min between two objects that can be separated. For optical wavelengths, this gives  min = 11.6 arcsec / D[cm]  min

28. Slide 28 Interference and diffraction

29. Slide 29 Resolution and Telescopes (SLIDESHOW MODE ONLY)

30. Slide 30 The Powers of a Telescope (3) 3. Magnifying Power = ability of the telescope to make the image appear bigger. The magnification depends on the ratio of focal lengths of the primary mirror/lens (F o ) and the eyepiece (F e ): M = F o /F e A larger magnification does not improve the resolving power of the telescope!