Sticking tape on a frosted glass makes it see through. Air and glass have a very different refractive index, so if you have a rough glass surface, the incoming light is scattered in all directions, thus blurring the image you see through it.
Tape has a similar refractive index as glass, so if you stick it to frosted glass, the sticky material will fill out the little bumps in the glass. The non-sticky side is practically flat, so by sticking tape to the frosted side of glass that’s frosted on one side, you are essentially making it flat again, and making the glass clear.
(Source: sixpenceee)
