Solar Engineering a Condensed Course

26Chapter 4Radiation Transmission Through Glazing; Absorbed RadiationDB Ch 54.1 Reflection of RadiationDB 5.1The reflectance r is given by Fresnel's expressions for reflection of unpolarized light passingfrom one medium with refractive index n 1 to another medium with refractive index n 2 :r ⊥ =r // =2sin ( θ2−θ1)2sin ( θ + θ )212tan ( θ2−θ1)2tan ( θ + θ )21(4.1.1)(4.1.2)wherer =IIri1=21sinθ1n2sin(r ⊥ + r // ) (4.1.3)n = θ (Snell's law) (4.1.4)2At normal incidence (θ = 0):n1= 1 (air) and n = nr( 1 20) = =I ⎜ ⎟ i ⎝ n1+ n2⎠2:2I ⎛ n − n ⎞r⎜(4.1.5)2Ir ⎛ n −1⎞r( 0) = = ⎜ ⎟ (4.1.6)I ⎝ n + 1 ⎠iExercise 4.1.1Calculate the reflectance of one surface of glass at normal incidence and at θ = 60°. Theaverage index of refraction of glass for the solar spectrum is 1.526.A slab or film of transparent material has two surfaces:1r(1-r) 2 r1-r(1-r)retc.(1-r) 2Figure 4.1.1. Transmission through one nonabsorbing cover.