What happens when you have your eyes dilated?
Dilating drops make your pupil bigger by working on the iris muscles. There are two types of chemicals that accomplish this (they are often used together). One type directly stimulates the iris muscles that open up the pupil. This is a very similar chemical to the one in our bloodstream that does the same thing in response to excitement or to seeing pleasing things: epinephrine.
Another medical chemical in a drop temporarily inhibits the iris muscles that constrict the pupil. You can see how these would work together.
Another effect of this latter iris muscle paralyzing drop is that it temporarily incapacitates your accommodation: the ability of another eye muscle surrounding the natural lens to "squeeze" it into the right shape for focusing up close.
Depending on the type of medical drop, its concentration, and how often it was used, the effect may last anywhere from an hour or less to several days!