White light fractures into its component hues when it passes through a prism or water droplet, revealing the intricate order hidden within what appears to be a single beam. This phenomenon occurs because light is composed of a spectrum of wavelengths, and each wavelength corresponds to a specific color that the human eye can perceive.
The Science Behind Visible Light
To understand what colours make a rainbow, one must first grasp the nature of visible light itself. The electromagnetic spectrum encompasses a wide range of wavelengths, but only a small segment is detectable by the human eye, typically ranging from approximately 380 to 700 nanometers. Within this window, shorter wavelengths appear as violet and blue, while longer wavelengths manifest as red and orange.
Dispersion: The Separation of Color
The process that creates a rainbow is called dispersion, which happens when light slows down and bends, or refracts, upon entering a medium like water or glass. Different wavelengths bend by slightly different amounts; shorter wavelengths curve more sharply than longer ones. When a beam of sunlight encounters a spherical water droplet, the light refracts, reflects off the inner surface of the droplet, and refracts again as it exits, spreading out into the distinct bands of color that define the spectrum.
The Sequence of the Spectrum
The classic mnemonic ROYGBIV provides a reliable guide to the order of colors in a rainbow, moving from the outer edge to the inner edge of the arc. This sequence represents the gradual change in wavelength across the visible spectrum, and it is consistent whether observed in a skyborne arch or a laboratory setup.
| Color | Approximate Wavelength |
|---|---|
| Red | 620–750 nm |
| Orange | 590–620 nm |
| Yellow | 570–590 nm |
| Green | 495–570 nm |
| Blue | 450–495 nm |
| Indigo | 420–450 nm |
| Violet | 380–420 nm |
Continuous Transition vs. Discrete Bands
While the name ROYGBIV suggests sharp divisions between colors, the spectrum is actually a continuous gradient. The boundaries between red, orange, and yellow are subtle, whereas the shift from blue to violet is more distinct. The human visual system interprets these varying wavelengths as a smooth transition, even though we classify them into named segments for convenience.
Atmospheric Conditions and Rainbow Formation
The specific environmental factors required to see a rainbow add another layer of complexity to the question of what colours make a rainbow. The sun must be positioned behind the observer, and water droplets must be present in the atmosphere in front of them. Rain is the most common medium, but spray from waterfalls, fog, and even dew can produce similar effects.
The geometry of a rainbow dictates that the light is always reflected at an angle of about 42 degrees from the direction opposite the sun. This fixed angle is why rainbows appear as arcs rather than complete circles, although full circular rainbows can be viewed from elevated vantage points such as mountains or aircraft, where the ground does not obstruct the lower half of the phenomenon.
