Why do colors change as it gets darker? Why do we associate dark blues with pitch black night? For the answer, let's look at the Purkinje Color Effect.

We've all seen it in movies. The sun goes down behind the hills. The shadows get longer and larger. Soon it will be pitch black onscreen, and the audience won't be able to see the popcorn in front of them.

Wait, no.

Movies are a visual medium, and when an auteur is filming a night scene, not a lot of filmgoers are going to sit still in a 100% dark theatre and hear the actors make noises for an hour (unless it's The Blair Witch Project, naturally). Instead, the filmmakers lower the lights a little and tint them blue. The actors all blunder around like it's actually dark, which I imagine must feel pretty ridiculous. Why should the audience accept the idea of blue light as low light?

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The answer, of course, is this matches up with the audience's own experience. As darkness falls, the reds and the oranges fade away and the blues become more pronounced. It's the result of the Purkinje Color Effect.

The retina of the eye has two different light-sensitive components. Rods, which are the more numerous, are more light sensitive in general, but have no receptivity to color. Cones, on the other hand, are few and weak, but brings the color into life.

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As the light dims, the cones lose their grip on our vision and the rods take over. The rods are most sensitive to shorter wavelengths, and — while the cones are more sensitive to the longer wavelengths — they lose those first as the light dims. The lower the intensity of the light, the less red and yellow we see. Eventually, the color disappears altogether, and all we see is black and white.

Apparently, it is because of the Purkinje Color Effect that submarine are lit in red. Red light allows minimal energy output for maximum engagement of both the rods and the cones in the human eye, which means that the crew can see in the dark but still read and discern details. And here I thought it was just because they wanted to blend in with the Commies.

[Via The Academic Dictionary.]