This morning, a beam of light coming through the window and through a prism cast a rainbow/spectrum on me, as I could see in the mirror. But when I positioned myself so that I was just intercepting the red part, the light coming from the prism still looked bright white. I had believed (misconception?) that a prism breaks up white light into components that have different wave lengths and that therefore the red part, say, would be made only of red wave length light. That’s what I anticipated seeing. Then I thought that maybe our eye lens reconstitutes the rainbow back into white light, but not if I was only seeing red light. I’m not sure what was happening and how my understanding is lacking. Can I get some help from the Dopers? xo,
C.
No, it’s not an illusion.
But, the eye has limited dynamic range. I would suspect that the light was so bright that the color receptors were saturated, so even though the light was red, it appeared white.
Rainbows are not optical illusions; the sunlight really is refracted and broken up into its different colors by droplets of water in the atmosphere - different wavelengths get refracted at different angles. Same with your prism. That’s also why rainbows can be photographed. As for your effect, beowulff’s hypothesis sounds quite reasonable to me.
Agree with first two answers. However, it’s also possible that you didn’t get your head in the right position. I find it hard to do. But when I have looked at the prism shining little spectra on the wall, I generally experience brilliant colored light, though it’s so dazzling that it is a little hard to see exactly what’s going on and I can’t look for very long to figure it all out.
I don’t understand what you mean by “intercepted.” I think I might need a diagram.
The fact that we see distinct colors is a result of our own perception, otherwise it’s just continuous wavelength not discrete. It’s not colored itself. So it’s something that’s not there as far as the radiometry is concerned, but it is not an illusion as our visual system is working fine in this situation.
The lens doesn’t do much with your color perception. It’s main effect is to focus light, but the optical density of it does filter out some wavelengths corresponding primarily to the violet-blue range.
I can’t provide a diagram for what I mean by “intercepted.” All I mean is that I positioned my head so that the red part of the spectrum fell on my eye, and I noticed that the source of the light appeared to be just white light, but quite bright. That, of course, was because I was looking at a projection of the sun’s light and that’s quite bright. But standing in the path of the spectrum should have had me seeing colors, or so I thought. Maybe, as beowulff suggested, the light was so bright that I could not detect the colors. (And still, I would think that an extremely bright red light would look red.)
There are a lot of variables here, but I suspect it’s just that you (anyone) cannot isolate the band that precisely. How big was the projection, how far from the window, were both eyes open? Also a big part of it is that the glass pane is reflective across much of its surface, while the light stimulus of the sun does not have a narrowly focused projection.
The luminance of a narrowband wavelength would not change colors, but some amount of other wavelengths from other locations on the pane could desaturate towards white, and it is not in the ability of a human to determine what the actual sources of light are.
Maybe my question is now refined - if I allow a rainbow/spectrum, or any part of it, from a prism to shine in my eyes, will I see colors?
Certainly.
We have a crystal chandelier that sparkles colorfully in the sun. If I stand in the path of the refracted light, I will occasionally see a brilliant flash of red or blue or green.
I have a prism set up on my windowsill, and in INDIRECT vision the colors from the beam are obvious.
Yes. If the source of the light isn’t too bright. Looking at a candle flame through a prism for instance will show you a flame with rainbow edges (where the refracted light isn’t overlapping). You either weren’t able to position yourself so that only red fell in your pupil (how wide was the red band at that distance? Did you try using a piece of paper or similar to observe the spread at the point in question?) or the brightness overloaded they eye. I don’t know if such an overload would be due to bright light overstimulating the rods, which do not distinguish colors, due to the width of the sensitivity of the cone cells, so that all colors were maxed out, or if vision processing somehow nulls out color information for very bright light, but there is no conceivable reason why your eye shouldn’t be receiving a narrow spectrum of red light if you place the pupil in a point where there is only such light as measured by placing a piece of paper there and observing the light reflected off it.
I don’t know what the OP is doing, but when I look into a hollow spherical glass ball filled with water (like a Florence Flask or something similar) at the appropriate angle relative to a light source I do indeed see a colored beam of light. The water-filled flask is a good model for a raindrop. If I let the light fall onto the screen it’s colored, too.
I also see colors when I look through a glass prism at the appropriate angle. In most cases, the light is not so bright as to dazzle my eyes so that I only see “white” light, and no color. I am baffled by the OP’s experience, because when I do what the OP says I do indeed see what they expect – bright color emanating from the prism.
Rainbows, as stated above, are definitely not an “optical illusion”. They are, however, not really a purely refractive phenomenon – the rainbow is the result of a diffractive phenomenon. A purely geometrical optics explanation of the rainbow gives the wrong expected intensity as a function of angle, and doesn’t explain the supernumerary bows. But that’s a whole different and more subtle topic than determining where the different colors come out.
According to the foremost authority on the subject, rainbows are visions, but only illusions, and have nothing to hide.