We can “see” light between 380 and 700 nanometers, but what happens at the egde and just after that? I mean, when we approach the limit like 699-700-701-705…
Does it turn completely dark/invisible/something else immediately or is there some kind of gradual change?
Is it the same on both ends?
In: 17
It just kind of gets darker and darker until we can’t see any light anymore.
We talk about being able to see 380-700nm, but our eyes are organic and the cutoff isn’t that exact. Some of our cells will be a _bit_ better as seeing light at the edge of the spectrum than others, so it will just appear darker and darker as there are fewer and fewer cells able to see the light.
Imagine if you couldn’t see green at all. Not that it looked gray, or the wrong colour, but it just didn’t register at all. You had photoreceptors for red, and photoreceptors for blue, but just none for green.
A colour that’s 100% red and 0% green, would appear entirely red. to all of us. A colour that was 50% red and 50% green, would appear as red with 50% luminosity to you, but yellow to the rest of us.
A colour that was 0% red and 100% green just wouldn’t appear. like 0% luminosity. But obviously very green to the rest of us.
Below red, and above violet, are exactly the same concept. Colours that we don’t have photoreceptors tuned to.
https://i.stack.imgur.com/vxjpm.png
Those are the sensitivity curves of the cones in our eyes. As you can see, they indeed just trial off.
It’s telling that not all people have equally sensitive cones, so some people can see a bit further into red and violet than others in the right conditions, like in a perfectly dark room. Because the light would be very dim in their eyes, even if the source is “bright” (energy wise).
If you graphed the spectral sensitivity of the average human eye, it’s not like a solid bar from 400-700nm. There are peaks in blue, green, and red wavelengths that rise up and fall off and overlap with each other. Sensitivity falls of to near nothing at the ends of the visible spectrum so most folks can’t see near UV or IR at all.
I do a bit of photography so I have various filters I use. The Hydrogen alpha filter I use to photograph emission nebulae allows only light at 656nm though, and it’s pretty dark to try to look though even out in the daylight because the eye isn’t very sensitive to longer wavelengths of visible light. My 720nm IR pass filter is completely opaque to my eye, but my modified camera will very gladly see through it.
You can look at a rainbow and see for yourself. Turn your hose on to a mist and spray it into the air during a sunny day. You will most likely need to spin around a bit to get the correct angle but there is a point where you will see the spectrum of colors like a rainbow.
> I mean, when we approach the limit like 699-700-701-705…
That is **exactly** what happens when you have a red-hot piece of metal that slowly cools down in an otherwise dark room. First it glows in the visible range, but as it cools down, the light increases in wavelength, until it gradually goes above 700nm and into the infrared range.
And while it still shines brightly in the infrared, to our eyes it looks like the glow slowly disappears into darkness.
It is the same on the other end, except I can’t think of an everyday examplpe of it right now, and also, you would probably suffer from subnurn in that case.
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