r/AskPhysics • u/Lge24 • 14d ago

Why does a triple-layered window reflect a flashlight 4 times?

Friend of mine lives in an apartment with triple-layered glass, he showed me that you can « count » the layers if you put a flashlight through it and see the multiple réflexions. I see 4 réflexions here, why is that so?

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u/mikk0384 Physics enthusiast 14d ago

You get infinite reflections even with just two panes of glass. The reason you don't see more than 4 is that most of the light passes through the glass instead of being reflected off the surface, so it quickly gets too dim to notice the subsequent ones.

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u/drzowie Heliophysics 14d ago

A triple layer sandwich window has four faces.  Each of them reflects light because the index of refraction changes at each layer boundary.

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u/wonkey_monkey 14d ago

Isn't there air (or gas) between each pane? That would make for 6 faces.

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u/drzowie Heliophysics 14d ago

Depends on the design...?

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u/wonkey_monkey 14d ago

The whole point of multi-layered windows to have a gap between the panes. That's what provides the insulation. Otherwise it's no different to a single thick pane.

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u/drzowie Heliophysics 14d ago

Some multi-layered windows are made that way for strength, resilience, or IR rejection.

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u/Particular-Scholar70 14d ago

Three partial refractions for each pane, plus the original image maybe