What is the source or cause of the Sun’s light?

Much like other stars the Sun is powered by nuclear reactions. The process of nuclear fusion takes place in the Sun’s core. The intense pressure and temperature at the core causes nuclear fusion - in this case hydrogen atoms fusing to create helium this process releases energy. The energy is emitted as gamma rays which travel slowly from the Sun’s core to the outer region. During the rise to the surface gamma rays collide with electrons degrading in temperature and energy to finally emerge  as visible radiation - this is the light Sun produces.