1	Chapter 5 Light: The Cosmic Messenger Chapter Outline Basic Properties of Light and Matter Learning from Light Collecting Light with Telescopes
2	Spectrum of Electromagnetic Waves
3	What is light? Since vision is a passive process, light must be something entering the eye that stimulates the brain to respond in what we call “vision” Two models (mental representations) for light Wave (continuous) model - light is a propagating wave that consists of varying electric and magnetic fields, a continuous phenomena called electromagnetic radiation Photon (discrete) model - light is discrete packets of energy moving from the light source to the eye, a discrete phenomena called photons or quanta
4	Speed of Light Ole Roemer (1644-1710) first evidence light moves at finite velocity c = 299,792 km/s (3 x 10⁵ km/s, 3 x 10⁸ m/s, 3 x 10¹⁰ cm/s)
5	What phenomena demonstrate that light has discrete properties? Energy carried by light is distributed discretely rather than continuously Energy content of photons Photon energy = (Plank’s constant) x (speed of light) / (wavelength of light) E_photon = hc/l Light is simultaneously a continuous wave and discrete photons
6	Phenomena that Demonstration Light Is A Wave Straight line propagation Reflection, refraction, and diffraction Superposition Inverse square law of light Doppler effect
7	Waves Wavelength – the distance between consecutive peaks or valleys in a wave form Frequency – the number of complete wavelengths passing a fixed point per unit of time Amplitude – the maximum or minimum excursion from undisturbed position in the wave form Wave velocity - product of the wavelength and the frequency
8	Superposition of Waves
9	Inverse Square Law
10	Electromagnetic Radiation
11	Spectrum of Electromagnetic Waves
12	Electromagnetic Spectrum
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14	Light’s Interaction with Matter Power – rate of energy transport or usage Unit for power is the watt: 1 W = 1 joule/second Emission – stimulated matter can emit radiant energy, i.e., transformation from some other form Absorption – matter completely diminishes radiant energy flowing through it, i.e., transformation to some other form Transmission – matter partially diminishes radiant energy flowing through it Reflection and scattering – matter changes direction of flow of radiant energy in either a predictable fashion or in a random fashion
15	Kirchhoff’s Laws of Spectrum Analysis
16	Absorption Spectrum of the Sun
17	Spectrum of Hydrogen
18	Laws of Thermal Radiation Hotter objects emit more total radiation per unit surface area per unit of time (Stefan-Boltzman law) Hotter objects emit photons with a higher average energy or shorter wavelength (Wien’s law)
19	Of what value are the thermal radiation laws in astronomy? Sun and stars are thermal sources of radiation Emit because they are hot Thermal radiation laws characterize the emitted radiation of a thermal source by temperature Can determine temperature for the Sun and stars
20	Do the spectra of different incandescent bodies look like that of a thermal source? No, the state of the body does influence the appearance of the spectrum of emitted radiation Thermal sources of radiation Non-thermal sources of radiation
21	Doppler Shift
22	What is the purpose of a telescope? A telescope collects, concentrates EM radiation, and in a particular wavelength interval forms an image, if possible, of the radiation source
23	Refracting Telescope
24	Yerkes Observatory Refractor
25	Reflecting Telescope
26	Reflecting Telescope Designs
27	Reflecting Telescopes
28	Telescope Mountings
29	What properties of a telescopic image should one care about? Size – how large is the image Brightness – how much radiant energy per unit area, i.e., how bright (light collected) is the image Angular resolution – how much angular separation exists in the image
30	Size of an Image Image size is directly proportional to the focal length, s µ l Longer focal length means larger images Shorter focal length means smaller images
31	Brightness of an Image Image brightness is directly proportional to the square of the ratio of diameter (d) of the main collector to focal length (l), b µ ( d / l )² A larger diameter means a brighter image A shorter focal length means that the image is smaller, but brighter
32	Angular Resolution in an Image Angular resolution (s) is directly proportional to ratio of wavelength of light (l)to the diameter of collector (d), s µ ( l / d ) A larger diameter means small angles can be seen A shorter wavelength of radiation means better angular resolution
33	What categories of analyzing instruments are used with telescopes? Image recording (cameras) - produce permanent image either photographically or electronically Spectrographs - disperse composite radiation to produce spectrum either photographically or electronically Photometers - collect radiation to make quantitative measure of amount of radiation in given wavelength interval, primarily electronic Radiation detector - common component of all analyzing instruments to detect EM radiation
34	Astronomical Spectrograph
35	Astronomical Photometer
36	What properties do radiation detectors possess? Sensitivity for a particular wavelength range of photons Sensitivity to range of numbers of incident photons Nature of response to photons Linear Non-linear
37	Properties of Radiation Detectors
38	Why put telescopes in space?
39	Atmospheric Effects – “Seeing”
40	148 ft Radio Telescope, National Radio Astronomy Observatory
41	The Very Large Array (VLA) Radio Interferometer
42	Entire Sky at Different Wavelengths
43	The Big Picture By dispersing light into a spectrum, we discover a wealth of information about the object from which the light has come. Most of what we know about the universe comes from information contained in radiation. Visible light is only a small portion of the complete electromagnetic spectrum. Different portions of the spectrum contain different bits of information about distant objects. By studying the spectra of a distant object, we can determine its composition, surface temperature, motion, and more. Telescopes collect, concentrate, and form images, if possible, of distant objects. New telescope technologies, along with adaptive optics and interferometry are making ground-based telescopes ever more powerful. Space-based telescopes and probes are an even more important platform for study of the universe.