The refraction of sunlight through water droplets is what causes the formation of a rainbow. When sunlight encounters a water droplet suspended in the air, such as during or after a rain shower, several optical phenomena occur.
Here's a simplified explanation of how a rainbow forms:
Refraction: As sunlight enters the water droplet, it slows down and changes direction due to the change in medium from air to water. This bending of light is called refraction.
Dispersion: The light is composed of different colors with varying wavelengths. During refraction, the different colors (wavelengths) of light bend by different amounts. This phenomenon is known as dispersion.
Internal Reflection: After refraction and dispersion, the light undergoes internal reflection within the water droplet. This occurs when light rays strike the inside surface of the droplet and reflect back towards the point of entry.
Exit and Further Refraction: Eventually, the light exits the water droplet. Upon exiting, it refracts again, bending and dispersing further.
Rainbow Formation: The dispersed and refracted light spreads out into a band of colors, forming a circular arc in the sky. The primary rainbow is the most commonly observed arc, characterized by red on the outer edge and violet on the inner edge. The exact angle at which the light exits the droplet determines the size and position of the rainbow.
It's important to note that the rainbow is a result of the collective effect of many water droplets in the air. Each droplet contributes to the overall rainbow, but we see a specific arc because of the way the droplets scatter and reflect light back toward our eyes.
The process described above is a simplified explanation, and there are additional factors that can affect the appearance of a rainbow, such as multiple internal reflections and the presence of secondary rainbows. Nonetheless, the refraction, dispersion, and internal reflection of sunlight through water droplets are the fundamental phenomena responsible for the creation of rainbows.