It's like a sunburn. The radiation damages the DNA of the cells. Cells are very clever, and know when their DNA is damaged, and almost always trigger a self-destruction mechanism when that happens. It's a cancer-prevention mechanism. So over the next few days as the skin or corneal cells start to go through their normal replication cycle, the DNA damage is discovered and causes those cells to die off. This causes an injury just like if another mechanism like heat or chemical damage, had killed those cells. Inflammation, oozing, pain, scarring.