Is light "pure" energy? I've been working out how much energy 60KG of material is equivalent to. However it strikes me as a possibility that I should not be using E=MC^2 because what I actually want to calculate is 60KG equivalent in light. Is there a factor I need to incorporate? I am trying to establish the consequences of being able to convert 60KG into light with 100% efficiency in terms of kilotonnes of TNT. Thanks very much!