Rainbow

A rainbow is a colorful circular arc caused by an optical phenomenon. This phenomenon occurs through the refraction, internal reflection, and dispersion of light within water droplets.

While we often think of rain, rainbows can also be formed by mist, spray, or airborne dew. To see one, the sunlight must shine from behind the observer at a low altitude angle. The rainbow appears in the part of the sky directly opposite the sun. This center point is known as the anti-solar point.

The mechanism of a rainbow involves several precise steps within a single water droplet. First, sunlight enters the droplet and undergoes refraction, which is the bending of light as it moves from air into water. The light then hits the back of the droplet and undergoes internal reflection. Finally, the light refracts a second time as it exits the droplet. This process causes dispersion, where the different wavelengths of light bend at different angles.

Because of this, blue light is refracted at a greater angle than red light. However, due to the reflection, blue emerges at a smaller angle to the original light ray. This results in red appearing on the outer edge and violet on the inner side of a primary rainbow.

There are different types of rainbows that an observer might see. The most common is the primary rainbow, which shows red on the outside and violet on the inside. Sometimes, a second, fainter arc appears about 10° outside the primary arc. This is called a double rainbow.

In a double rainbow, the colors are perceived in reverse order, with red on the lower side. This occurs because the center of the second arc is the sun, not the anti-solar point. Another rare type is the moonbow, or lunar rainbow. These occur on strongly moonlit nights. Because human vision is poor in low light, moonbows often appear white to our eyes.

Scientists have long studied the components of these colorful displays. Isaac Newton was a key figure in this history. In 1672, Newton originally divided the spectrum into five main colors: red, yellow, green, blue, and violet. He later added orange and indigo to create a sevenfold sequence.

He did this by analogy to the number of notes in a musical scale. Modern interpretations often divide the rainbow into red, orange, yellow, green, cyan, blue, and violet. The exact number of colors is somewhat arbitrary because the spectrum is a continuous transition.

The physics of the rainbow involves specific mathematical measurements. The most intense light of a primary rainbow is returned at an angle of approximately 42°. This angle depends on the refractive index of the liquid. For example, seawater has a higher refractive index than rainwater. This means a rainbow formed in sea spray will have a smaller radius than a true rainbow.

The light of a primary rainbow is also 96% polarized tangential to the arc. The light in a secondary rainbow is 90% polarized.

Rainbows can take different shapes depending on the observer's position. Most people see only an arc because the ground blocks the rest of the circle. However, if you are in an airplane, you may see a full circle.

You might also see a full circle near waterfalls or fountains. The sky inside a primary rainbow actually appears brighter than the sky outside. This is because each spherical raindrop scatters light over an entire circular disc. Red light is scattered over a larger angle than blue light, which creates the visible edge of the bow.

Understanding rainbows connects to broader ideas in physics and linguistics. The way we name colors may even change how we see them. Research suggests that the number of distinct colors observed can depend on the language a person uses. This is related to the concept of linguistic relativity. Furthermore, rainbows involve light frequencies beyond what humans can see. There are bands in the near infrared and ultraviolet regions. While cameras using black and white film can sometimes see the UV band, these remain invisible to the human eye.