2. Radiation Field Basics I Specific Intensity Ω = dddd cos d tA I E
2. Radiation Field Basics I Specific Intensity Ω = dddd cos d tA I E
2. Radiation Field Basics I Specific Intensity Ω = dddd cos d tA I E
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For sufficiently small ∆V, the intensity is homogeneous, so thetwo integrations (V, Ω) are independent. The energy density isu1= dΩcνI ν<strong>Radiation</strong> PressureEach photon has momentum p = hν/c. Component of momentumnormal to a solid wall per time per area isdpν=1 dEν<strong>cos</strong>θc dAdtRe-write in terms of I ν and integrating over solid angle gives:n p = hν/cθdAhν/c <strong>cos</strong> θds = c dtp1= <strong>cos</strong> 2θ dΩcνI νIsotropic radiation has p ν = u ν /3.<strong>Radiation</strong> pressure is analogous togas pressure, being the pressure ofthe photon gas.
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