E=mc^2 is not quite the culmination of his 10-year research, nor is it about light (but obviously light serves as a foundation); it is actually the beautiful theory of gravity that is now known as general relativity, which is formulated in the most elegant way from his field equation: G_mu_nu = 8*pi*G*T_mu_nu. There should be some c factor in it but natural units ftw. E=mc^2 is from one of his 4 papers released during his wunderjahre back in 1905, while his GR papers were in 1915.