slide 1 light and telescopes just by analyzing the light received from a star, astronomers can...
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![Page 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](https://reader031.vdocument.in/reader031/viewer/2022032521/56649d5f5503460f94a40276/html5/thumbnails/1.jpg)
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
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Slide 2
What is light?
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Slide 3
Electricity
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Slide 4
Magnetism
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Slide 5
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Slide 6
Electromagnetic induction
Time-dependent magnetic field creates time-dependent electric field, and vice versa
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Slide 7
Electromagnetic waves
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Slide 8
c
f
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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*108 m/s
• f and are related through
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Slide 10
Wavelengths and Colors
Different colors of visible light correspond to different wavelengths.
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Slide 11
The Electromagnetic Spectrum
Need satellites to observe
Wavelength
Frequency
High flying air planes or satellites
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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).
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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!!!
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Slide 14
Dual, wave-particle nature of light
c
hhfE sec Joule106.6 34 h
1 eV = 1.6x10-19 J
c = 3x108 m/s
1 Angstrom = 10-10 m
Speed of light in matter:
cm = c/n, wheren is refractive index
Note: n is a function of
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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!
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Slide 16
Optical TelescopesAstronomers use
telescopes to gather more light from
astronomical objects.
The larger the telescope, the more
light it gathers.
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Slide 17
Refractors and Reflectors
(SLIDESHOW MODE ONLY)
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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
Focal length
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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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Slide 20
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Slide 21
140-ft Hevelius telescope 1673
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Slide 22
Newton’s telescope: the first reflecting telescope
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Slide 23
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Slide 24
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Slide 25
Telescope parameters
• Light-gathering power (ability to see faint objects)
• Resolving power (ability to see fine details)
• Magnification (least important)
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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
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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
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Slide 28
Interference and diffraction
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Slide 29
Resolution and Telescopes
(SLIDESHOW MODE ONLY)
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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 (Fo) and the eyepiece (Fe):
M = Fo/Fe
A larger magnification does not improve the resolving power of the telescope!