What happens if I shine a white light on a black paint spot?

Answer

Dear Seth,

The particles in the paint that provide the color are called pigments. A pigment is a substance that absorbs certain wavelengths of light and reflects others. The reflected wavelengths then determine the color. For example, a yellow pigment mainly absorbs blue wavelengths. If you shine white light (for simplicity’s sake, think of white light as a combination of blue, green, and red wavelengths) on a yellow pigment, the blue light is absorbed and the green and red light are reflected back, which to us looks like yellow.

A black pigment is a special pigment because it absorbs most of the wavelengths. Only a small part of each wavelength is reflected, so it appears very dark to our eyes. Black is not really a color in itself. You can’t just mix different paint colors to make black paint. If you do that, you’ll mostly end up with dark gray or brown paint, because each particle of pigment still reflects some light. So for black paint you really need black pigment. The same, of course, applies to white paint and white pigment.

If you shine a strong white light on black paint, it will still look “black” to your eye, or at least dark. This is because most wavelengths are still absorbed. But in strong light you can also see that some light is reflected. Just look for a few black objects at home and put them in the sun: you will see that some objects look dark gray rather than black under that bright light. Black pigments are almost never perfect. One kind of black pigment that is almost perfect (almost no longer reflects light) is vantablack.

Regards,

Francis