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Blackbody defined…  A blackbody is an object that absorbs all light that hits it  Also emits light provided that its temperature is above absolute zero  com/HP/WCL/IMG/bbo dy.gif

A Blackbody…  Perfect “black body” – something which absorbs all the radiation that falls on it  Good absorber of radiant heat is also a good emitter  Main scientist , G. Kirchhoff  Foundation of blackbody radiation lies in the idea that radiation is released from blackbodies in the form of "quanta" or discrete packets of light called photons  Main scientist – 1900, Max Planck

More about a Blackbody…  Is the best possible emitter of radiant energy  Must both radiate and absorb energy at the same rate in order to maintain a constant temperature  Total radiation from a black body depends only on temperature of the body, not on chemical or physical characteristics

Plotting Curves  A curve can be generated plotting the temperature, intensity, or brightness of the black body versus the wavelength coming from it.  These curves are sometimes called Planck curves.

Blackbody curves, 4 objects a) Cool, invisible galactic gas cloud called Rho Ophiuchi.  Temperature of 60 K  Emits mostly low- frequency radio radiation  astron2e/AT_MEDIA/CH03/ CHAP03AT/AT03FG13.JPG

Blackbody curves, 4 objects b) A dim, young star (shown here in red) near the center of the Orion Nebula.  Temperature of star's atmosphere ~ 600 K  Radiates primarily in infrared (IR)  astron2e/AT_MEDIA/CH03/ CHAP03AT/AT03FG13.JPG

c) The Sun  Surface ~ 6000 K  Brightest in the visible (v) region of the electromagnetic spectrum  astron2e/AT_MEDIA/CH03/ CHAP03AT/AT03FG13.JPG Blackbody curves, 4 objects

d) A cluster of very bright stars, called Omega Centauri, as observed by a telescope aboard the space shuttle  Temperature ~ 60,000 K  Radiate strongly in ultraviolet (UV)  astron2e/AT_MEDIA/CH03/ CHAP03AT/AT03FG13.JPG

How is a star a blackbody?  Because blackbody radiation is solely dependent on temperature (simple)  And to maintain a constant temperature, a blackbody must emit radiation in the same amount as it absorbs

Wein’s Law  The hotter a blackbody becomes, the shorter its wavelength of peak emission becomes  The wavelength of peak emission is simply the wavelength at which a blackbody emits most of its radiation

Wein’s Law  1893, German physicist Wilhelm Wien  Quantified relationship between blackbody temperature and wavelength of spectral peak  λ max = 2.9 x (microns)/T  λ max (lambda max) = wavelength of Peak emission  2898 microns  T = temperature of Blackbody in Kelvin (K)

Wein’s Law in action…

Plank Curves - 1  1900, Max Planck  Electromagnetic radiation absorbed or emitted only in “chunks” of energy, quanta, E  Quanta are proportional to the frequency of the radiation E = h. (Constant of proportionality “h” is Planck's constant.)  Wanted to understand the shape of Wien's radiative energy distribution as a function of frequency. 

Plank Curves - 2  Postulated that radiators or oscillators can only emit electromagnetic radiation in finite amounts of energy of size.  At a given temperature T, there is not enough thermal energy available to create and emit many large radiation quanta.  More large energy quanta can be emitted when temperature is raised. 

Plank’s Law  The amount of blackbody radiative flux emitted by a blackbody for a given wavelength is given by Planck's Law:  Where T is object temperature (in degrees Kelvin); l is wavelength in microns; units are (W/m 2 ) per micron  The wavelength of peak emission is:

Stefan–Boltzmann Law  Independently formulated by Josef Stefan (1879) and Ludwig Boltzmann (1884, 1889)  Relationship between radiant energy and temperature for a black body radiator  Relates total radiant flux (F) (in W/m 2 ), from surface of black body to its temperature (T)  F= σ T 4  σ = x watt / m 2 K 4

Stefan–Boltzmann Law 2  How much power a blackbody radiates per unit area of its surface  For a blackbody of temperature T, the power radiated per unit area is:  P = constant x T 4  ~imamura/122/images/st efanboltzmanlaw.jpg

Stefan–Boltzmann Law

Why use Stefan-Boltzmann(S-B) Law?  Using the Stefan-Boltzmann law in conjunction with other known quantities, it can be used to infer properties of a star  For example, if a star radiates like a blackbody, then the luminosity of the star can be written as  L = (Surface Area of the Star) x (power per unit area produced by the star) = 12.6 x R 2 x constant x T 4 So, if we know certain information (obtained through independent means) about a star, we can infer other properties. For example,

What can we learn from S-B law?  If we know the luminosity and temperature, we can infer the radius of the star;  If we know the luminosity and radius of a star, we can infer its temperature;  If we know the radius and temperature of a star, we can infer its luminosity

Blackbody Review  Stefan-Boltzmann Law - Area under the curve increases as the temperature is increased  Wien's Law – Peak of the curve in emitted energy changes wavelength  Planck’s Law – Peak of the curve or the peak emission wavelength of a blackbody is related to the temperature of the object – hotter objects emit in higher wavelengths.