Black Body Radiation Flashcards
What does it mean if an object (not a star) is red?
Absorbs all light except red photons, which it reflects.
Why is a star like Betelgeuse red?
Emits red photons.
What is a black body?
A perfect absorber/emitter, absorbs all intercepted photons.
What emits EM radiation?
All objects with T > 0K
What is thermal equilibrium?
The matter and radiation field have the same temperature.
What do typical black body curves look like for different temperatures (intensity plotted against wavelength)?
High temperature looks like negative skewed graph, and gets lower with the peak further to the middle as temperature decreases.
What is the equation for black body intensity at λ as a function of T?
B(T) = 2hc^2/λ^2 * 1/(exp(hc/λkT)-1)
What is the equation for the quantisation of energy?
E = hc/λ
What is the first property of the Planck function?
Area under the black body curve gets larger as you move to higher temperatures.
What is the equation for radiance?
integral from 0 to inf of B(T) dλ = σT^4/π
What is the definition of radiance?
Total energy radiated per unit surface area of a black body across all wavelengths per unit time per steradian.
What is the second property of the Planck function?
The black body peak wavelength moves bluer with temperature.
What is Wiens law?
λmax*T = const
or
log λ = -log T + C
What does the log black body curves graph look like?
Lines going up and then turning and going down, moving higher and to the left with temperature.
What is a solid angle?
3D analogue of an angle. Units are steradians.
What do we need to compute to relate intensity and flux?
Triple integral over all angles to get total intensity (luminosity)
What is the equation for luminosity in terms of black body intensity?
L = 4πR^2πB(T) = 4πR^2*σT^4
How do we relate the black body intensity with flux?
Use the two luminosity equations and set them equal to each other.
What is the integral equation for the black body intensity?
B(T) = integral from 0 to inf of B(T) dλ
What is the effective temperature and what is its equation?
The temperature that the star would have if it were a black body. F = σTeff^4
How many temperatures are there for which a blackbody emits the same amount of energy as the star? What does this imply?
One temperature, implying that black body curves do not cross eachother at all, hence black body curve has a unique temperature.
Give an example of a black body radiator.
A cavity radiator (heatable cavity with small aperture).
