Question

What is the emissivity of aluminum foil?

(a) 0.03

(b) 0.9

(c) 0.02

(d) 0.04

I had been asked this question by my college professor while I was bunking the class.

My doubt stems from Miscellaneous Properties in portion Properties of Engineering Materials of Engineering Material & Metallurgy

Answer 1

Right choice is (a) 0.03

To elaborate: The emissivity of aluminum foil at room temperature is 0.03. The emissivity of polished silver and copper is 0.02 and 0.04 respectively, while that of concrete/brick is 0.9. Since emissivity is the ratio of the total emissive power of the body to the total emissive power of a perfectly black body, it does not have a unit.