# Tag: refraction

## A light bending exercise… in space!

By Phil Plait | June 26, 2012 10:17 am

On board the International Space Station, ESA astronaut André Kuipers just put up this ridiculously cool and fun picture of himself playing with water in space:

Wheee! But what are you seeing?

SCIENCE!

Let me explain.

The really short version of this is that the water is acting like a lens, flipping his face over. But there are two images of André’s face in there, and one is upside down! What gives?

First we need to look a the drop itself. On Earth, sitting on a surface like a tabletop, water drops tend to be flattened. But in space, where gravity’s not an issue, water drops form little spheres. That’s because of surface tension, an imbalance in the electromagnetic forces between water molecules, and is a whole post all by itself! But for now, what you need to know is that in orbit where there’s no net effect from gravity, water droplets form little balls.

In this case, you can see the drop isn’t a perfect sphere; it’s big enough that it can oscillate like a spring, elongating in one direction. That’s cool, but doesn’t affect what’s going on here too much — it just elongates the image of his face seen in the drop a little bit.

But we’re not done! The astronauts injected an air bubble into the drop. On Earth, that bubble would rise and pop, but again, when gravity isn’t your master, the bubble stays put. So in the middle of the water drop is an evacuated sphere filled with air.

So what’s with the funhouse mirror stuff?

Ah, that’s because light can be bent! When a beam of light passes through water or some other transparent material, the direction it’s traveling changes, which is why a spoon sitting in a glass of water looks like someone bent it. This is called refraction, and depends on two different things: the material itself (different stuff bends light by different amounts) and the direction from which the light hits it.

The shape of the refracting material — the lens — also changes the image we see coming from the source. The curvature of the lens affects the direction the light is bent. In the case of light coming from outside a sphere of water, the light hitting the top of the drop gets bent down, light hitting the right bends left, and so on.

And one other thing: the path of light bends whenever it passes from one medium to another, so it bends if it’s going through air and then hits water, and it also bends if it’s going through water and hits air!

So now we can figure this all out. Read More

CATEGORIZED UNDER: Cool stuff, Pretty pictures

## Sunsets are Quite Interesting

By Phil Plait | November 20, 2011 7:14 am

There’s a wonderful comedic quiz show in the UK called "QI" — for "Quite Interesting" — which is hosted by none other than Stephen Fry. The participants are comedians, and they’re asked questions ranging over just about every topic you can imagine. The BBC recently uploaded a clip about which alert BA Bloggee Brett Warburton informed me. In it, Fry shows the contestants a video of the Sun setting, and asks them to ring in when they think the Sun has completely set. Here’s the clip:

This is, in fact, correct! The Earth’s air bends the image of the Sun upward, so we can still see the Sun even though it is physically below the horizon. If we didn’t have air, daytime would be shorter. In fact, this effect works for sunrise as well, so we see the Sun rise before it’s physically cleared the horizon.

And Stephen was correct in the amount too; the light is bent upward by just about the same size as the Sun, so when the lower limb of the Sun just kisses the horizon it’s actually already set.

But it’s a bit more complicated, of course. Read More

CATEGORIZED UNDER: Astronomy, Cool stuff, Humor, illusion

## Squishy moonrise seen from space!

By Phil Plait | January 21, 2011 11:30 am

Check. This. Out: Moonrise as seen by astronaut Paolo Nespoli on board the International Space Station!

Holy wow! Click to spacestationate.

That is so cool. As the ISS races around the Earth at 8 km/sec (5 miles/sec), it sees up to 18 sunrises and sunsets each day, and the same number of moonrises and moonsets. Paolo had to snap quickly to get this sequence, which couldn’t have taken more than a minute to elapse.

But what’s with the squished Moon? Here’s a closeup of the Moon in the three pictures:

What causes this? It’s an atmospheric effect, due to the air surrounding the Earth acting like a lens, bending (or, if you want to impress your friends, refracting) light. You’ve probably seen how a spoon looks bent when it sits in a glass of water, right? Same thing. Light passing from the vacuum of space through our air gets bent a bit. The amount of bending depends on how much air the light is going through; the thicker the air the more it’s bent.

When an astronaut on the ISS sees the Moon near the Earth’s limb, as in these shots, light from the top part of the Moon is passing through less air than the bottom. So the light from the bottom gets bent more, in this case, up. This makes it look as if the bottom of the Moon is being squished up into the top, like a clay ball that’s been dropped on the ground. As the ISS orbits the Earth, and the Moon gets higher off the limb, the effect diminishes so in the two subsequent shots the Moon gets re-inflated.

You’ve probably seen this yourself, though not as dramatically. The next time you have a clear horizon view to a sunset (like maybe on the west coast, as the Sun sinks below the waters of the Pacific) you’ll see exactly this same effect. The Sun will look squashed. I’ve actually posted about this a couple of years ago, when a similar picture of the Moon from the ISS was released. It wasn’t as dramatic as this one, though!

Paolo Nespoli has a Flickr page where he posts amazing pictures he’s taken from space. You could have a much worse Friday than clicking through some of those shots and seeing how lovely our world is when seen from above.

Image credit: ESA/NASA. Tip o’ the spacesuit visor to Stuart at astronomyblog.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
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