A rainbow is an illusion created by the reflections of many raindrops each sending light to your eye at a specific angle.
Drag the sliders or pull on the marked raindrop below to see how the reflections change and relate uniquely to an observer.
What an Observer Sees
You are viewing the scene from the Sun's perspective. An observer is fixed in-line between the sun and the antisolar point that every rainbow is centered on. Drag the sliders to update the scene. Drag the raindrop to see its own, individual reflected cone (dashed rings) and whether its light is visible to the observer. A rainbow is formed by the interaction of many raindrops sending different light signals.
Why This Angle and Color?
Incoming light's reflections disperse in different directions, but the geometry of a spherical drop focues most reflections at a specfic angle (~42° for one internal reflection and ~51° for two internal reflections). This focus angle is called a caustic. Each color/wavelength of light exits at a slightly different focus angle, helping to spread the light into distinct rainbow colors.
Side View
The observer is fixed with their head at the horizon. Drag the droplet to see if its reflections return a colored rim, the washed out pale zone, or Alexander's dark band. Sliders here are independent of the panels above.
Why Caustics Form
The exit angle of reflections isn't spread evenly. The geometry of the droplet causes the a wide range of entry locations to focus at specific exit angles. That focusing creates the caustic: the rainbow's bright edge. Violet, green, and red dip at slightly different angles since they have different wavelengths, and that separation is what makes the rainbow colorful. Drag the slider to trace one entry point and see how caustics form.
The Visible Spectrum
The colors that make up a rainbow are different wavelengths of visible light. Shorter and longer wavelengths are invisible to the human eye.