$E=mc^2$ is the famous mass-energy equation of Albert Einstein. I know that it tells that mass can be converted to energy and vice versa. I know that $E$ is energy, $m$ is mass of a matter and $c$ is speed of light in vacuum.
What I didn't understood is how we will introduce speed of light?
Atom bomb is made using this principle which converts mass into energy; in that the mass is provided by uranium but where did speed of light comes into play? How can speed of light can be introduced in atom bomb?
Answer
c is a priori not the speed of light. It is the speed of massless particles. The way it comes about is as follows: You construct the Lorentz-transformations as the symmetry transformations of Minkowski space. The group has one parameter, that's c. You have to fix it by physical means. You can look at the dynamics of massive particles and massless particles and find that massive particles will approach c asymptotically only at infinite energy, and massless particles always move with c.
Since to our best knowledge photons are massless, c is also the speed of light. Also, that was historically Einstein's motivation, which is why it's usually motivated in textbooks this way. However, should it turn out one day that photons do have very tiny masses, then c will still be there, it will just no longer be called the speed of light.
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