So you’re hanging out with friends and being all cool and casually taking a sip of water,
and then somehow, you miss your mouth … and slop it all down your front.
Now, not only are you wet, but everybody knows exactly how uncoordinated you are, because
there’s a dark patch on what used to be a nice red shirt.
Well, for that you can thank physics -- the physics of light.
When light hits something, the light bends at an angle.
We’ve talked about this before when it comes to mirrors -- or, depending on how you like
to check yourself out: maybe spoons.
In the case of mirrors, though, the light bounces back right toward you.
It’s like throwing a ball at a wall.
When light bounces back in your direction, it’s called reflection, but when light gets
deflected away from you -- that’s refraction.
And just to keep things interesting, the light can also split up, with some of it being reflected
and some of it being refracted.
Some of it can also be absorbed, too -- It just depends on the material.
So armed with this knowledge, let’s talk about your shirt, or more specifically, the
light that’s travelling through air and hitting the fabric.
While the fabric is dry, some of the light is being absorbed by it, while the rest of
it is reflected back at your eyes, so you see it as red.
Then you spill.
Now there’s a layer of water clinging to the fabric, and that’s what the light is
hitting first.
Just as before, some proportion of the light is being reflected off the water, but the
rest is refracted into the water.
Basically, it keeps going.
But then, it hits another barrier: the fabric.
And again, some of that light is reflected, while the rest is absorbed by the fabric.
But the reflected light doesn’t just head straight back toward your eyes.
I mean, it starts to travel in your direction, but once it gets to the surface of the water,
some of the light goes through what’s called total internal reflection, which sounds really
intense.
Instead of escaping the water and continuing its journey toward your face, it bounces back
into the water for another round.
And this extra trip back into your shirt means that a larger proportion of the light gets
absorbed by the fabric, which makes it appear darker.
So now you know why your shirt turned a darker shade of red when you spilled.
Doesn’t change the fact that you’ve still got this embarrassing stain, but hey, at least
you didn’t get your pants.
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