Yup, it is kind of an optical illusion, and "they are of same length" is the correct answer.

But it would be interesting to hear what are people's first impressions when they take a look at the picture - even when they know that the lines actually are equal, but how do the lines appear if you don't think about it too much?

This kind of a figure has been used in an empirical test, revealing some pretty interesting results. But before telling more about it, I'll wait for a couple of days to see if we get more reports from different people =)