"Spectacles" is a more formal way of saying "glasses". You can compare this difference to "shades" and "sun glasses". However, a spectacle can also be referred to as a visually striking display. Like a dance show or concert, a sight that will leave an impact on the viewer. Your listener / conversation partner will understand based on the context. (on the street with your friend and he just put on his glasses/spectacles): "Those are some sweet spectacles man" vs. (a man is starring out over a destroyed city): "That spectacle will be forever burned into my head" I hope this helps :)

Interchangeable, but glasses is used much more commonly these days in the US

There is no difference. 'Spectacles' is more unusual, and slightly old-fashioned.

Oh really? I didn't know that! Thank you!

"Glasses" in the sense of очки are also called "eyeglasses".