When Harry tries on the Invisibility Cloak for the first time, you can only see his head. What would you see if you sat next to him and looked up? [duplicate]

Question

Would it be the inside of his neck? The bottom of his jaw? A big, back hole where his insides should be?

Answer 1

Similar to the disembodied hand in the library later what you would see is the nearest object behind your point of view that is not covered/obscured by the cloak (after all the cloak does really make you invisible by showing what would be seen if the person wearing the cloak was not there).

If you sat next to him and looked up it would depend on your vantage point what you would see. If the cloak would cover everything between your POV and the visible part you would still not be able to see anything - just like if you stood behind him in this picture:

Only from the side can you see the hand - and the lamp - everything else is covered by the cloak.

Essentially it is exactly how it is done in the movie: imagine a green/blue blanket that gets keyed out with the image behind it. If the blanket is between the camera and object the object is completely hidden.