Since diodes emit light(led) and they are also used to convert ac to dc

[jaydnul]

Are they shining brightly inside the device when used to convert ac?

 

[Bobbywhy]

All diodes do not emit light, only the type called "light emitting diodes" do. Visit this Wikipedia page and find a description of at least EIGHT different types of diodes:

http://en.wikipedia.org/wiki/Diode

Cheers,
Bobbywhy