At the back of the drop

Most of it just leaks out

The colored light has reached the back of the drop. Here about 94% of it goes straight through the back and out. Only about 6% reflects like a mirror and heads back inside. The rainbow is made from this small amount of light.

Why?For light to be trapped and reflect completely, it would have to hit the back wall at a slant of more than 49°. The light that makes the rainbow hits at about 40°, which is not slanted enough. So, just like your faint reflection in a window at night, only part of it reflects.

Learn moreComplete reflection is called total internal reflection. Many explanations say that light is totally reflected inside a raindrop, but the light that makes a rainbow is not. The reflected share comes from the Fresnel equations. The lines on screen are brightened so you can see them; the real shares are the numbers in the labels.

Light reflected back inside

about 6%

Angle at the back wall

about 40°

Needed to reflect completely

more than 48.6°

calculated from the refractive index of water (IAPWS, 20 °C) and the laws of refraction and reflection