Who Can Tell Me What "Optical" Means?

A tween-age friend of mine has recently become quite email savvy, which means I'm treated to lots of e-chain letters from her. You know, the kind where you have to scroll through names and address of past recipients for several minutes until you finally see a picture of a kitty and dog wearing sunglasses, or something similar?

The latest of these was a series of optical illusions, and I have to admit, they're pretty fun. I seem to recall sending and receiving these exact same optical illusion emails back when I was in junior high and my friends and I had just gotten our first email accounts, with ridiculous names like: rockstargrrl123@juno.com or luv2lol84@aol.com. Perhaps these bizarre types of "forwards" are part of some developmental phase one goes through upon forming an online identity... but any way, on to the fun.

In case you can't read the text, it says: "focus on the dot in the centre (are these from England?) and move you [sic] head backwards and forwards. Weird hey...")

Are the purple lines bent or straight? (The world may never know...)

Do you see gray squares in between the black ones?

Stare at the image for 20 seconds or so, then stare at a blank wall. What do you see? Or should I say, who do you see?