How can those wearing the One Ring be seen in the sunlight as a shadow?
[Bilbo] could see outside into the open air; there were a few steps running down into a narrow valley between tall mountains; the sun came out from behind a cloud and shone bright on the outside of the door -- but he could not get through.
Suddenly one of the goblins inside shouted: "There is a shadow by the door. Something is outside!"
The Hobbit - page 89 - Del Rey Paperbacks - chapter 5, Riddles in the Dark
If the one wearing the One Ring is invisible, how is it they can cast a shadow in the sunlight? Does the ring not induce true invisibility, or is the invisibility similar to a Chameleon charm in Harry Potter, where a person just blends in exactly with his/her surroundings and cannot be seen because he/she looks exactly like his/her background; however, they're not technically invisible, so I suppose the sun could cast a shadow in that instance.
How can those wearing the One Ring be seen in the sunlight as a shadow?
I'd like an answer based in Tolkien canon if possible.