Solve the below:
Q: The directional total emissivity of nonmetallic materials may be approximated as εθ, = εn, cos θ, where εn, is the normal emissivity. Show that the total hemispherical emissivity for such materials is 2/3 of the normal emissivity. You must provide the solution of given problem step by step.You should include all the units which are needed to solve the problem