When you look at letters on paper through a glass slab, they appear slightly raised because of refraction.

Refraction occurs when light travels from one medium to another, such as from air into glass. Light moves at different speeds in different materials.

When light moves from the paper through the glass slab to your eyes, it bends, making the letters seem closer to the surface than they actually are. This creates an illusion that the letters are raised, even though they’re at the same place on the paper.