Taken in Strasbourg, France, features grey clouds framing an astounding rainbow with seven fainter rainbows trailing below it. This infrequent effect, called a supernumerary rainbow, comes about when the raindrops generating a rainbow are particularly uniform in size. As the different light waves spread out, they interfere with one another and form areas of darkness or brightness. These appear as pastel fringes below a regular rainbow.
For 14 years, the website Optics Picture of the Day has collected and featured stunning examples of optical effects. We look at some of our favorites and the physics underlying them.