She says: It’s better to light a candle than to curse the darkness
He says: I wouldn’t be so sure, maybe a candle would destroy your night-vision – without the candle your eyes could adjust to the lowered light levels (a process called adaptation, [Hack #26])
She says: But if you’re in total darkness, there’s no light at all to adjust to seeing
He says: Good point, so maybe it should be “It’s better to wait for a bit, then, if your eyes don’t adjust, you should light a candle rather than curse the darkness”
She says: How long do you have to wait until you know?
He says: Ah well, the cone receptors in the eye – which let you see colour – adapt fully after about 5 minutes. But it takes about 30 minutes for the rod receptors to fully adapt. These are the important ones for night vision, since they are specialised in detecting light or dark – which is presumably the fundamental information you are interested in.
She says: Okay. So it should be “It’s better to sit in the dark for up to 30 minutes doing nothing, then light a candle rather than curse the darkness”?!
He says: Oh, you don’t have to do nothing. Adaptation happens at the retina. You can prove this to yourself by adapting to the dark and then looking at a light with only one eye. One eye will adjust to the light, and the other (which you kept closed) will keep it’s dark adaptation. Now if you go back to darkness you can switch between being blind (in your light adapted eye) and being able to see (in your other eye), just by openning and closing your eyes alternately. So, you can do anything you want with the rest of your brain, it shouldn’t matter.
She says: So talking would be okay?
He says: Talking would be fine. Or whistling.
She says: So “It’s better to wait in the dark to see if your eyes dark adapt (you can do anything you want while you’re waiting) and only then, if they don’t, light a candle rather than curse the darkness”
He says: You could even curse the darkness while you’re waiting and get it out of the way. And really a red light would be better than a candle, because red spectrum light doesn’t affect your dark adaptation (which is why cabin lights in aeroplanes and ships are red).
She says: “It’s better to wait in the dark to see if your eyes dark adapt (you can do anything you want while you’re waiting) and only then, if they don’t, light a candle rather than curse the darkness. But it would be better if you had a red light rather than a candle for preference”
He says: That’s it
She says: Snappy. I like it
He says: Someone should tell Amnesty