Presentation on theme: "3RF Sciences, LLC. Blackbody defined… A blackbody is an object that absorbs all light that hits it Also emits light provided that its temperature."— Presentation transcript:
3RF Sciences, LLC
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
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.