Unformatted text preview: Example: Compare photons close to and far from an extended source like
a star:
star photons here are crowded in physical
space (high density) but spread out
in solid angle (direction) photons here are diffuse in
physical space (low density)
but highly collimated in
angle (high density). So, dilution of photon density is always accompanied by increased collimation in a precise quantitative relationship! 3.1.3 Brightness
Let's compute the total energy dE in a small volume of phase space dE = EdN = E N d3 x d3p 1 E 2 dE d =
c3
= E 3N =c2 d2 dt dE d E N (cd2 dt) Id2 dt dE d :
Here we have dened I = N E 3=c2 :
We see that I is the energy in photons per (normally incident) area per
time per solid angle per photon energy interval dE. I is called the \specic
intensity" or \brightness" of a radiation eld at a particular point, in a
particular direction, at a particular energy.
29 ...
View
Full
Document
This note was uploaded on 12/29/2011 for the course AST 350 taught by Professor Dion during the Fall '09 term at SUNY Stony Brook.
 Fall '09
 dion
 Space

Click to edit the document details