Transcript: We can’t see into the Sun. The Sun is opaque like a frosted pane of glass. Opacity or optical depth is the degree to which a material transmits light. If a material transmits all of the light incident on it, it is transparent, and its opacity or optical depth is zero. If it transmits none of the light it’s opaque, and its opacity or optical depth is high. In the Sun, radiation suffers collisions with atoms or ions that exist there at high temperature and very high density. The frequent collisions make a high opacity which means the radiation cannot travel freely. It’s the difference between having a town square where you walk across it freely because no one else is there and a town square full of a crowd of people where you would have to jostle your way through colliding many times before you could cross the square. In one case the opacity is zero. In the other case the opacity is high. In the Sun the opacity is high enough that we cannot see inside the surface’s few thousandths of a kilometer.