There's no mystery there. If you note the effect, the colors remain in the same places. Some don't show up, that's all. When you broaden the
slit, white light can swamp the greens (or whatever colors are being blocked).
As far as the "combination" of colors, there are actually two different methods. One is subtractive, the other additive (as you might have
guessed). When you mix pigments (pains, crayons, etc.) the combinations are subtractive. Red is red because it absorbs colors other than red,
reflecting mostly the red. So if blue falls on it, most of it gets absorbed. Same with green, etc.
If you mix, say, blue and red paint, you get a darker color, a purple. The blue reflects mostly blue light, the red reflects mostly blue light, and
what's left appears purple.
Subtractive color mixing
But if you *add* blue and red light, you get a lighter color, not a darker one- it's kind of a light magenta.
Additive color mixing
This is the kind of stuff they teach in photography and art classes. It's well-known, and there is no mystery about it.
As for the prism, look where the light is hitting it. When it hits near the point, the light comes out red-orange-yellow; when it's towards the
base, it's blue-violet. Again, no mystery.
You might want to invest in a small prism and play around with it for a while, to explore just how they work. Hey, it' a cheap evening of fun if the
Cable's out.









