What does the sun's interior look like?

Firstly, the inside of the Sun is opaque - it is rather like asking what the inside of a brick looks like. There is visible light emitted, though, especially at the outer layers before the atmosphere thins enough to become transparent. In the core your prof is mostly right, most of the energy is actually produced as gamma waves. It has to be absorbed and re-emitted several times to be reduced down to visible wavelengths. However, it is a chaotic system and enrgy distributions are not uniform, so you would still expect a good amount of themal emission in the visible wavelengths.