Light and telescopes : Light and telescopes Just by analyzing the light received from a star, astronomers can retrieve information about a star’s Total energy output
Surface temperature
Radius
Chemical composition
Velocity relative to Earth
Rotation period
What is light? : What is light?
Electricity : Electricity
Magnetism : Magnetism
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Electromagnetic induction : Electromagnetic induction Time-dependent magnetic field
creates time-dependent electric field,
and vice versa
Slide7 : Electromagnetic waves
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Light as a Wave (1) : Light as a Wave (1) Light waves are characterized by a wavelength l and a frequency f. f = c/l c = 300,000 km/s = 3*108 m/s f and l are related through l
Wavelengths and Colors : Wavelengths and Colors Different colors of visible light correspond to different wavelengths.
The Electromagnetic Spectrum : The Electromagnetic Spectrum Need satellites to observe Wavelength Frequency High flying air planes or satellites
Light as a Wave (2) : 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).
Light as Particles : 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!!!
Slide14 : Dual, wave-particle nature of light 1 eV = 1.6x10-19 J c = 3x108 m/s 1 Angstrom = 10-10 m Speed of light in matter: cm = c/n, where
n is refractive index Note: n is a function of
Stars are hopelessly far away … : Stars are hopelessly far away … Matter in space consists of the same atoms as matter on Earth
Physical laws should be the same
Optical Telescopes : Optical Telescopes Astronomers use telescopes to gather more light from astronomical objects. The larger the telescope, the more light it gathers.
Refractors and Reflectors : Refractors and Reflectors (SLIDESHOW MODE ONLY)
Refracting/Reflecting Telescopes : 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 Focal length
Disadvantages of Refracting Telescopes : 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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Slide21 : 140-ft Hevelius telescope 1673
Slide22 : Newton’s telescope: the first reflecting telescope
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Slide24 :
Telescope parameters : Telescope parameters Light-gathering power (ability to see faint objects)
Resolving power (ability to see fine details)
Magnification (least important)
The Powers of a Telescope: Size Does Matter : 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 = p (D/2)2 D
The Powers of a Telescope (2) : 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. amin = 1.22 (l/D) Resolving power = minimum angular distance amin between two objects that can be separated. For optical wavelengths, this gives amin = 11.6 arcsec / D[cm] amin
Slide28 : Interference and diffraction
Resolution and Telescopes : Resolution and Telescopes (SLIDESHOW MODE ONLY)
The Powers of a Telescope (3) : 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 (Fo) and the eyepiece (Fe): M = Fo/Fe A larger magnification does not improve the resolving power of the telescope!