Immagine you are looking at an electric light bulb, not bright about 25 watt.
What is happening? Electricity is passing through the wire causing it to heat up and energize the ions in the gas inside the bulb. The ions emits electromagnetic radiation (mysterious stuff), some of which, a small part, manages to enter your eyes and strike your retinal nerve cells. This causes another reaction where the electromagnetic energy stimulates the production of electro-chemical impulses which travel along your optic nerve in the black box called your brain, eventually arriving at the vision center where something mysterious happens and you “see” the light bulb. So far so good. What is also clear is that until the electro-chemical impulse reaches the vision center in your brain, you have not seen anything, the light from the bulb does not enter your head, it stops at your retina.
So, if whatever the light bulb gives off does not enter your head, where does the “light” you see come from?
Now think about looking at the sun.
Now think about all the light you have ever “seen” in your life.
This is called swallowing the sun.