What is in the eye drops used to dilate your eyes in eye examinations? how do they work?

Anticholinergic: Dilating eye drops, commonly Tropicamide & cyclopentolate, act by blocking muscarinic receptors in the iris sphincter muscle thereby blocking cholinergic input which causes the pupil to dilate. This effect is termed Anticholinergic, and the effect that makes the pupil smaller (miotic) is termed Cholinergic. There is also an effect on accommodation which is why dilation affects your near focus.