“Compare red nail polish to red ink: both are red, but the nail polish will be visible on black paper because it reflects light. The ink won’t be, because it absorbs light.”
doesn't seem right to me, but I'm not an expert. The ink doesn't reflect light? If you put a drop of red ink on a sheet of glass, it won't look red? Isn't the difference because the ink will soak in to the paper while the polish will sit on top of it?
The nail polish is opaque, which means it reflects or absorbs light that hits it. Red nail polish reflects red light and absorbs the rest. Shine white light on it, and it will look red.
Ink is transparent; It lets through or absorbs light that hits it. Red ink lets though red light and absorbs the rest. Shine white light through it, and it will look red.
Shining white light on a black piece of paper with red ink on it will look black. What light is not absorbed by the ink is absorbed instead by the paper.
Of course red ink reflects light, as you say, but that is just because of reality. It is not essential to its function. If it were possible to make a 100% non-reflective ink, it would work perfectly well.
It does. Thanks to you and the other excellent replies, I now finally understand how ink works. Apparently it's a filter, and we see the light after it has been twice filtered by the ink, down through the ink into the paper and reflected from the paper back up through the ink to our eyes. In contrast to paint or polish, which reflects from its surface.
Ink mainly changes the color of light that passes through it. Nail polish absorbs and reflects light at its surface. If you look at a deep pool of red ink, it looks pretty close to black because it mostly absorbs light instead of reflecting it. The wording could be clearer though.