Saturday, September 10, 2022

Why are red lighs OK?

Why are red lights ok when viewing penguins? Humans have three types of cone cells which have peak sensitivity to blue light (S-cones), green light (M-cones) and red light (L-cones) - see graph below. It is our red-sensitive L-cones that allow us to see red when we use red light at night. The retinas of most animals, however, do not have L-cones which means they are almost oblivious when we illuminate them using red light. Red light is the best (and recommended especially) for nocturnal viewing of animals as it does not affect their night vision. As you can see from the graph below they can see some of the red spectrum, hence we ensure we have animal-safe torches that have wavelengths that do not disturb the penguins. 

Penguins evolved to not have L-cones presumably because the red light wavelengths are the first ones to be filtered by water, and hence the colour red is not as relevant to their survival, as infrared is not relevant to our survival. From: "Colors are really nothing more than different wavelengths reflected by an object. Underwater, waves travel differently, and some wavelengths are filtered out by water sooner than others. Lower energy waves are absorbed first, so red disappears first, at about 20 feet. Orange disappears next, at around 50 feet. Then yellow at about 100. Green stays longer and blue the longest, which is why things look bluer the deeper you go. As long as the water is clear, that is. In murky water there is less light penetration and things tend to look greenish-yellow. What this means is that if you're diving at 60 feet or so, you see mostly blues and greens. Yellow hangs around, but it's muted. No more red or orange."

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