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Comment by chris_engel | Hacker News Reader

A blue plastic bag filters most of the spectrum EXCEPT the blue part. Not the other way around. Simple proof: look inside, the light is blue.

I guess the biggest problem for the plant here is that especially the infrared part of the sunlight is very important. Yellow leaves usually mean not enough light. Especially tomatoes need lots of sunlight.

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Nope. If the bag looks blue that means it's scattering away blue light while letting the rest through. So it's filtering (out) the blue. OP is right.

If you see blue on BOTH the outside of the bag and the inside, what does that mean? I’ve never thought of this.

It means some blue goes through, some is reflected.

As a guess, perhaps the unfiltered light that gets through goes through a phase change from being reflected. https://en.wikipedia.org/wiki/Reflection_phase_change

No, a blue filter, like a blue plastic bag would let blue pass and absorbe the other colors. That's why if you shine a white light through a blue filter, things look blue on the other side.

Now, some of the blue light is scattered away, and that's why you see the plastic bag itself as blue.

Hang on, so if you shine a white light through a blue filter, the light on the other side looks blue. And the filter itself still looks blue on both sides. But then, where did the non-blue component of the light go?

Converted to heat. What's going on here is a discussion about transmission and transmittance. [1]

[1] https://www.swiftglass.com/blog/key-differences-transmission...

Imagine looking at this blue-transparent bag with a red light, it will appear more opaque.

Just like laser light protection goggles.

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