Alejandro L. answered 10/26/16
Chemistry tutor (all areas) + Intro Astronomy
I must start by saying that your instructor doesn't understand what it is that he/she is teaching because light has no mass and much less weight. I presume that the misunderstanding of what Einstein's equation predicts is the reason for this kind of question.
Technically, E = mc^2 only applies to mass particles, but for the sake of this problem we'll pretend that it applies. However, I do want to make sure that you understand this calculation is completely meaningless and has no physical truth to it whatsoever.
First determine the energy of orange light using Planck's equation:
E = hc/L
now substitute into Einstein's equation:
mc^2 = hc/L
solve for m:
m = h/cL = [(6.626x10^-34 Kgm^2/s)/(2.998x10^8 m/s)(5.90x10^-7 m)] = 3.75x10^-36 kg or 3.75x10^-24 ng
Once again, this is the answer to your question using completely misunderstood equations.