Rainbows always form at a 42-degree angle from the point directly opposite the sun.

The 42-degree angle of a rainbow is a result of the refractive index of water and the geometry of a sphere. When sunlight hits a spherical raindrop, it undergoes refraction as it enters, total internal reflection at the back of the drop, and refraction again as it exits. Isaac Newton and René Descartes were among the first to mathematically describe this phenomenon in the 17th century. Descartes used ray tracing in 1637 to calculate that the maximum concentration of light is returned at an angle of roughly 42 degrees.The specific angle actually varies slightly by color because different wavelengths of light bend at different amounts, a process known as dispersion. Red light reflects at approximately 42.4 degrees, while violet light reflects at 40.7 degrees, which is why red always appears on the outer edge of the primary arc. The center of this circular arc is the antisolar point, which is the imaginary point exactly 180 degrees away from the sun relative to the observer's head.Because the rainbow is a geometric projection, no two people see the exact same rainbow. Each observer sees light from a different set of raindrops that happen to be at the correct 42-degree orientation for their specific location. If the sun is higher than 42 degrees above the horizon, the antisolar point is so far below the horizon that the 42-degree cone cannot reach the observer's eyes unless they are at a high altitude, such as in an airplane.