The sign for glasses (or eye glasses/spectacles) looks like you are outlining an imaginary pair of eye glasses on your head. Taking your thumb and index finger on each hand, start with them open around your eyes, then bring the thumb and index finger together while pulling your hands to either side of your face. It's similar to the ASL sign for cat, but the latter is formed around your cheeks, while glasses is formed around your eyes.
An alternative ASL sign for glasses is to simply curve your index fingers and thumbs around either eye at the same time, forming the shape of the lenses of the glasses.
If you wear glasses, it is probably something that your baby is curious about and always wants to touch. Get your baby his own pair of toy glasses that he can wear and play with. "Let's wear our glasses." "Daddy is wearing his glasses."
We love to teach this sign in association to Mom's eye glasses. The sign for glasses also proves useful if your baby needs glasses at a very early age.