Light and the
Telescope

(Section Not Complete)

Most of what we know about the Universe comes from information that has been carried to us by light. Thus, it is appropriate to begin our study of the Universe by examining the properties of light and the instruments that are used to detect it.

The Wave Properties of Light

In modern physics, light or electromagnetic radiation may be viewed in one of two complentary ways: as wave, and as a stream of massless particles (photons). It is sometimes useful think of it as a wave and sometimes useful to think of it as photons. The quantity that is "waving" is the electromagnetic field, an esoteric but quite measurable entity. As illustrated in the adjacent image, a wave has a wavelength associated with it. It is common for light to give the wavelength of light the symbol Greek lambda.

Lectures on Telescopes

Java applet: inverse square law

Java applet: Blackbody Radiation

The visible spectrum


Continuous, emission, and absorption spectra


Separation of light by a prism according to wavelength


Hydrogen Balmer Spectrum

Type of Radiation
Approximate Wavelength
A
Microns
Gamma rays
0.01
10-6
X-rays
1
10-4
Ultraviolet
1000
0.1
Visible
5000
0.5
Infrared
1 x 105
100
Microwave
1 x 106
103
Radar
1 x 107
104
Radio
1 x 109
106

(See http://instruct1.cit.cornell.edu/~tlh10/lec04.htm)

Hydrogen spectral lines

Hydrogen 21 cm transition

Map of neutral hydrogen in galaxy using 21 cm line

Ionized helium

Notation

Suffix
Meaning
Examples
I
neutral
He I, O I
II
once ionized
He II, O II
III
twice ionized
He III, O III

(See http://instruct1.cit.cornell.edu/~tlh10/lec06.htm)

Lecture: Blackbody Radiation

Blackbody spectrum

Refracting Telescope

Reflecting Telescope

Cassegrain Focus

Newtonian Focus

Radio Telescope

Interferometer

Atmospheric Windows

Wavelength
Characteristic Object
Gamma-Ray
Compact object which collapsed
X-Rays
Neutron stars
Ultraviolet
Hot stars, quasars
Visible
Stars
Infrared
Red giant stars, galactic nuclei
Far-IR
Protostars, dust, planets
Millimeter
Cold dust, molecular clouds
cm Radio
HI 21-cm line, pulsars

(See http://instruct1.cit.cornell.edu/~tlh10/lec12.htm)

Refraction of Light

The direction of light propagation can be changed at the boundary of two media having different densities. This property is called refraction, and is illustrated in the following figure for the boundary between air and water.

Refraction of light


The apparent and actual positions of the fish differ because the direction of light propagation has been changed as light passes from the more dense water into the less dense air. Such effects form the basis of the refracting telescope, and of optical devices using lenses in general.

Diffraction of Light

Because light is a wave, it has the capability to "bend around corners". This is called diffraction, and is illustrated in the adjacent image. The intensity of light behind the barrier is not zero in the shadow region.

This has a number of consequences for astronomy. Two of the more important are that this property is the basis for the diffraction grating that can be used to separate light into its constituent colors, and that diffractive effects set an absolute limit on the quality of an image observed through and optical instrument such as a telescope because the lenses of such objects are of finite size and cause diffraction of light waves.

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