Does the rainbow have seven colors? The short answer is “it depends,” and the long answer is more physics than tradition. The rainbow appears when sunlight enters the water droplets, changes direction, reflects inside them and comes out again. Each wavelength, that is, each light wave, is deflected in a different way as it passes through the water, and thus the colors that arrived mixed in the white light are separated.
What does not exist are seven bands with physical borders. Between red and violet there is a continuous transition of hues, and that is why, if you look closely at a rainbow, you will see that the colors intermingle instead of being perfectly separated. The light is not divided into seven watertight compartments.
The figure seven comes from Newton, who popularized the division into red, orange, yellow, green, blue, indigo and violet. These names help remember the order, but they do not force nature to draw seven stripes: counting colors is a convention about what physically happens when light passes through a drop of water.
The video adds a detail to pay attention to the next time one comes out. In the main arch, red is on the outside and violet is on the inside. If a second arc appears, the order is reversed, because the light has been reflected twice inside the drops and its path changes.
Santiago Campillo (@Scruzcampillo) is a biologist and scientific communicator specialized in health, nutrition and nature.
