Why is light always refracted by the surface between water and air, not the content?

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I accidentally noticed this several days ago, as I was playing with a transparent plastic bottle in the bath tub. I'm not fully sure why it happens, so I thought it would be interesting enough to ask:

Consider you have a thin transparent plastic bottle, like a coca-cola recipient. If it's empty and you are looking through it in the air, it does not noticeably refract any light. But if you fill it with water and look through it again, you see everything deformed through the bottle. This is probably not surprising, since water is much more dense than air... but the reverse also applies: If you submerge the bottle underwater while it's filled with air, it will distort what's behind it. Whereas if you fill the bottle with water and submerge it, you don't see anything deformed.

Of course, this shouldn't come as a surprise to me. Since if you're standing outside a pool of water and look at something located inside, you will see that object deformed... and if you're submerged underwater and look at something above the surface, you will again see it deformed once again. But no deformation occurs if the eye and the object are both in the air or both underwater.

Still, how does it work exactly... and why does light refract only when passing through the surface between air and water? Even when it passes through two such surfaces, like the transparent bottle.

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It is because the speed of the wavefront in a medium depends on the refractive index. The light is slowed down and this causes it to change direction.

(Just the first image I found.)

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The light is slowed down and this causes it to change direction.

Or sped up.

You are less likely to notice this with a thin sheet, especially if it's flat, because the amount of lateral shifting depends on the thickness, i.e. the empty bottle only has two sets of small shifts. But if the bottle is full of something that's a different index, then the displacement of beams happens over a greater distance, and the curved surface ensures you will notice the distortion — the curve will likely be different for entrance and exit.

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Still, how does it work exactly... and why does light refract only when passing through the surface between air and water? Even when it passes through two such surfaces, like the transparent bottle.

Because refractive index of air is ~1.0, while refractive index of water is ~1.33.

If you will have two transparent materials with equal or very similar refractive indexes, there will be no distortions, and you will have no idea that something is there, object will "disappear".

You can see this experiment on video

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Thank you, I understand better now.

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It is because the speed of the wavefront in a medium depends on the refractive index. The light is slowed down and this causes it to change direction.

(Just the first image I found.)

Nonsense. You have no idea what you're talking about.

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Moderator Note

winger,

If you can't post anything productive, kindly don't bother. Your attitude is not welcome here.

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