Ie if we had a light that could emit light from purple to UV. Do we see an instant change from purple to nothing or does it slowly fade out?
In: Physics
If you mean decrease as in the wavelength gets shorter then the color of the light would shift moving from red at the longer wavelengths through the rainbow spectrum down to violet and eventually to ultra violet and your eyes would see nothing. If it was the only light source it would become dark. Unfortunately if the light kept shifting and/or you kept looking for a long while it would begin to damage your eyes.
If you mean decrease as in get longer, the opposite would happen, as the light would get redder and ultimately fade out and become dark again. This light wouldn’t damage your eyes like UV because it’s lower and lower energy.
it would seem to fade. in fact, the apparent brightness of the light would change *dramatically* as it went through the spectrum because our sensitivity to certain wavelengths is highly varied.
It fades out. You can see this yourself from the diffraction colors on a DVD, provided you’re using a light source that includes the frequencies of interest.
I’d imagine it would be similar to watching something glowing red hot to just emitting infrared radiation. Just look at a piece of wood in an oven, when the fire has nearly died out, blow some air on it and see it start to glow again, and then it stops after a while.
I have actually seen this, since I worked in a lab with a laser tunable to well into the UV. For tests we would run it through the whole spectrum. In addition, this method is interesting because the output brightness stays roughly the same (the number of photons is mostly conserved).
What happens is that you see the light source shift gradually along the rainbow: green then blue then violet. And then the violet smoothly but quickly fades and then you see nothing.
As another poster mentioned, in theory you should perceive different brightnesses during the sweep, but in practice you’re looking at light “leaking” from the photon splitting crystal in a small chamber with uniform walls, so you don’t really notice it.
here you can see a video of a green laser pointed at low temperature, when it’s very cold it becomes IR light, as it heats up it become more and more green
[https://youtu.be/9tOcUyakk0Q?t=204](https://www.youtube.com/watch?v=9tOcUyakk0Q)
I’ve seen this at the near-IR end of the scale a lot, because I used to deal with really high powered near-IR lasers. You can absolutely see near IR if it’s bright enough, even up to like 780 or 800nm. The problem is if it’s bright enough for you to see it, it’s dangerous. It’s always a “fuck I forgot my goggles” moment if it happens. (it just looks red, naturally)
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