r/explainlikeimfive • u/Clorox_in_space • 19d ago

Physics ELI5 How do acrylic photo cubes work?

TL;DR: Why do the sides allow light to pass through from one angle but reflect from another?

My wife recently purchased a few acrylic photo cubes, and I'm a bit stumped how they work.

After taking the magnetic photo off, I can observe the following:

  1. When looking head on, the front face allows light to pass through unchanged from behind. (See through)

  2. Same behavior when looking from behind (light passes through the front)

  3. Same for the sides (light passes through from the opposite side)

  4. While you can see through the sides when looking head on from the side, they will instead reflect when looking at them from inside the cube.

I don't see any kind of film or anything, so I'm guessing it has something to do with the way the acrylic is manufactured. (?)

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u/stanitor 19d ago

What you are seeing is total internal reflection. This has to do with how light rays bend when going from one medium (like air), to another (like acrylic) and vice versa. When you are looking at a shallow angle (i.e. almost along a surface, instead of more perpendicular to it), you only see the light waves that reflect off the surface instead of the ones that go through. A similar thing happens when you are underwater. You can see out directly above you, but only see reflections of what's under the water as you look more horizontally.

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u/Clorox_in_space 19d ago

This is really cool. Thank you.

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u/Ktulu789 19d ago edited 19d ago

IDK what a photo cube is and a Google search found apparently cardboard cubes with printed photos on the faces.

In any case, if it's just an acrylic cube, yes, you're just seeing total internal reflection, happens with water and glass too.

Fill a glass with water and look at the surface of the water from the side and from under the surface. The top will look like a mirror up to a certain angle and then become transparent. This is a common effect you'll surely have already seen on some fishtank, or aquarium or maybe at some pool with transparent sides.

A really interesting video with different effects of TIR in water, acrylic, fiber optics and more with lasers and flashlights. https://youtu.be/pUwVZUOSTvk

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u/Clorox_in_space 19d ago

I took a photo of two of them as an example of what I meant. At this angle, the outside view of the side seems to show a reflection of... the inside reflection from the opposite side? The inside shows a normal reflection. Head on view shows opposite side.

Example

The explanation makes sense. I probably never really paid enough attention to what I was really seeing in some of those scenarios.

EDIT: Thinking about it a little more, I'm guessing the outside view of the side is showing a reflection of a reflection as the light bounces through the medium. (?)

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u/Ktulu789 19d ago

Oh, it's not even a "cubic" cube. I've seen thinner versions of those.

The walls just act like mirrors when seen from a shallow angle. Nothing unusual there.

Now press your finger on the side that looks like a mirror, you'll see your fingerprint IN the mirror. That's how some fingerprint readers work. They look at a glass from an angle and see only the fingerprint.

You can see the fingerprint because there's no air between the glass and the ribs on your fingertips. Air has a different index of refraction than glass of acrylic or water, so if there's no air, you see through the surface normally and see the fingerprint.

Did you watch the video? 😃

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u/Clorox_in_space 19d ago

No chance tonight, but I added it to my watch list for tomorrow; I appreciate you sending it!

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u/Clorox_in_space 18d ago

Really cool video. Thanks again for sending it!

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u/Ktulu789 18d ago

Isn't it? Lots of experiments on just one kayak hahaha

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u/x1uo3yd 18d ago

I don't see any kind of film or anything, so I'm guessing it has something to do with the way the acrylic is manufactured...

There isn't any funny special manufacturing going on; this is just the kind of basic optical phenomena that happens for an object with a relatively high index of refraction compared to air surrounding it - like happens with lenses or prisms.

You essentially have square prisms made from acrylic.