 Hi, this is example 5.5.1 from page 214 of the Duffy and Beckman text The previous problem we just did was for a single cover system This problem is about two covers and the glass panes that make up this two cover system have a kl 0.03 70 per pane absorber of this system Has an al absorptivity or alpha equal to 0.90 if you're not familiar with my notation a w with a slash means width and then alphas the absorptivity there We are asked to find The tau alpha product. Let me just back up a bit So that I can make that clear as mud the tau alpha product 50 degrees angle of incidence So this problem the best way to start it off is using Figure 5.3.1 from the text Where you can find from that figure that at But the transmissivity at that incidence angle of 50 Is simply 0.75. So that's an approximation using that figure and then the next really important piece of the puzzle is to look at Figure 5.5.1 Has some footnotes so look at footnote 2 footnote number 2 figure 5.5 Point and this figure gives a really great representation of What's happening inside of a cover absorber system where the light is is reflecting back and forth within the system and And you're trying to figure out what the overall transmissivity and absorptivity product is So from that footnote we can see that With a KL of 0.03070 Rho sub D the reflectivity Sub D is 0.22 So that's pretty important Once we know those two pieces this zero point seven five and zero point two two we can then use Equation five point five point one To calculate the tau alpha product using these values so What you end up with is zero point seven five Times zero point nine zero, which is the Absorptivity Divided by one minus one minus zero point nine zero times zero point two two all of that in the denominator And so you get a tau alpha product when you crunch those numbers of zero point six nine and this is Essentially a property of the overall system. It's not it's not simply the product of tau times alpha There's it's a little bit more involved and it has to do with the inter reflections within the system as well But this can be this is a very useful metric to help Assess one system against another when you're in in light of the Transmission properties and the absorption properties of the materials in the system. So thanks for listening This has been example five point five point one from the text