How much load? Center loaded or uniform?

This is the only place I go when designing shelves.

http://www.woodbin.com/calcs/sagulator.htm

A 50 pound (uniform) load on your size with .25 glass shows a .10 deflection (.09 would be noticeable) Bump to 5/16 and it is .05, 3/8 is only .03.

Personally with that span I would go with 5/16 at a minimum unless there was some kind of center support available.

Joe

ETA: uniform loading