$($a$)(3$pts$)$ What is the definition of absorption coefficient $?$ What is it related to W TOT $($the total transition rate$)?($b$)(2$ pts $)$ What is the equation for $?($In terms of $Eg,p_{cv},$ and $$ and other physical constants$).$

$($c$)(5$pts$)$ Sketch the function $()$ vs$.,$ for the range of $>E_g.$

$($d$)(5$pts$)p_{cv}$ is not dependent to $.$ When $()$ has the maximum, what is the value of $?$