The Emerald Glasses are the glasses worn by the citizens of the Emerald City, who wear them all the time when about the city.
In the Novels[]
Wicked[]
It is believed that they are worn to protect the citizens' eyes from being damaged by the bright green wonder of the Emerald City. This is most likely linked to the storyline of The Wizard of Oz in which the citizens are forced to wear the glasses to make them believe that the city is emerald green, a trick brought about by the Wizard. They are also used as a way to stop them from seeing what is actually going on around them.
In the Musical[]
The city is actually green, and the glasses are made quite superfluous. The citizens and visitors wears the glasses in order to protect their eyes from the brightness of the light of the city. There is someone who gives out the glasses when visitors arrive in the city, including Elphaba and Glinda.