If Mass Can Be Converted Into Mass Then Energy Can Be Converted Into Energy. Therefore Light is a Source of Energy What is the Mass of Light

It is indeed a good question. Even though light has no rest mass it does have an effective mass which has all the properties one expects from MASS - in particular, it has weight in a gravitational field and exerts a gravitational attraction of its own on other masses. The classic Gedanken experiment on this topic is one in which the net mass of a closed box with mirrored sides increases if it is filled with light bouncing back and forth off the mirrors. Further, you can refer through a good search engine or from a good physics text book on light.