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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?. Electricity. Magnetism.

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Light and telescopes l.jpg
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





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Electromagnetic induction

Time-dependent magnetic field

creates time-dependent electric field,

and vice versa



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Light as a Wave (1)

l

c = 300,000 km/s = 3*108 m/s

  • Light waves are characterized by a wavelength l and a frequency f.

  • f and l are related through

f = c/l


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Wavelengths and Colors

Differentcolors of visible light correspond to different wavelengths.


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The Electromagnetic Spectrum

Wavelength

Frequency

High flying air planes or satellites

Need satellites to observe


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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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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*sis the Planck constant.

The energy of a photon does notdepend on the intensity of the light!!!


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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 


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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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Optical Telescopes

Astronomers use telescopes to gather more light from astronomical objects.

The larger the telescope, the more light it gathers.


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Refractors and Reflectors

(SLIDESHOW MODE ONLY)


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Refracting/Reflecting Telescopes

Refracting Telescope: Lens focuses light onto the focal plane

Focal length

Reflecting Telescope: Concave Mirror focuses light onto the focal plane

Focal length

Almost all modern telescopes are reflecting telescopes.


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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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Telescope parameters

  • Light-gathering power (ability to see faint objects)

  • Resolving power (ability to see fine details)

  • Magnification (least important)


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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:

D

A = p (D/2)2


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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.

Resolving power = minimum angular distance amin between two objects that can be separated.

amin = 1.22 (l/D)

amin

For optical wavelengths, this gives

amin = 11.6 arcsec / D[cm]



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Resolution and Telescopes

(SLIDESHOW MODE ONLY)


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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!