XKCD

[Portia]

http://xkcd.com/1145/

What is the answer to the rollover text?!?!

[Giovanni Schwartz]

in my mind, it's because it's that the mirror reflects whatever is closest to it. So, the top of the Y-axis is still the top of the mirror. #notveryscientific

[Eirene]

When you hold a piece of paper up to the mirror, you've already reversed it horizontally yourself by spinning it around (think about trying to read a piece of paper that you're holding up facing a lamp--you can also make it vertically flipped if that's the way you flip the paper over). The only thing the mirror does is reverse images from front-to-back, perpendicular to its surface (so if you have a bunch of objects lined up in front of the mirror, their mirror image order will be reversed so that whatever was in the back of the room in reality will be towards the front of the "mirror room").

[UffishThought]

The way I think of it is that the arrow of reflection isn't traveling left to right, or up to down, but straight forward. But it took me some thinking to figure that out.

[Digit]

As for the main strip's question about why the sky doesn't look violet, according to this, it's not that the violet wavelengths aren't getting scattered, it's to do with human cone cell physiological perception.

the sky's combination of violet and blue elicits the same cone response as pure blue plus white light, which is an equal mixture of all the colors.

"Your eye can't tell the difference between that complex spectrum and one that is a mixture of pure blue and white," Smith said.

So the sky is blue and violet, but everybody sees it as blue and white.

[Emiliana]

Thank you thank you thank you for these answers! It bugged me all morning...

[Dead Cat]

I just want to say that I thought today's comic was hilarious.