The Science of Rainbows - Introduction

These pages are my notes on how rainbows and related phenomenon occur. Note that this is a passion project made by an amateur; there may be typos and some information may not be accurate due to flaws in my understanding.

These pages use JavaScript for light/dark mode buttons and to keep the page navigation consistent for all pages.

visable spectrum diagram. Indigo starts at approx. 375 nanometers, going through the colors of a rainbow before ending in red at 700 nanometers

The Visible Spectrum

The visible spectrum is a portion of the electromagnetic spectrum. The electromagnetic spectrum organizes particles of energy (photons) by their wavelengths and amount of energy. Photons travel at the speed of light in waves. The wavelength of these photons determines their properties (hence the separation of the electromagnetic spectrum into categories). Light and heat are forms of energy (the visible spectrum and ifrared radiation). X-rays, microwaves, UV-rays, and radio waves are other forms of energy made of photons.

diagram of a wave. the wave is a curvy line with rounded peaks and valleys. the wavelength is labeled as the distance between two peaks

diagram of the electromagnetic spectrum. From smallest to longest wavelength: Gamma rays, X rays, ultra violet, visable spectrum, infrared, microwaves, and radio waves.

Because they all travel at the same speed, the shorter the wavelength, the higher the frequency of waves, thus the higher the energy.

The photons (energy) in the visible spectrum take the form of light. Infrared radiation (radiation = emission of energy) is heat. Visible light, infrared radiation, microwaves, and radio waves are forms of nonharmful radiation, as the photons do not have enough energy to break chemical bonds or ionize molecules. UV radiation, X-rays, and Gamma rays are harmful to living cells because the photons do have enough energy to cause chemical reactions. This is why you wear a lead cover when getting a dental X-ray, as X-rays can't pass through lead. The sun emits UV radiation, which is why sunscreen is important.

red, orange, yellow, green, blue and violet light depicted as waves, illustrating how red has the widest wavelength and violet has the smallest

Each wavelength (380-750 nm) corresponds to a different color. Violet wavelengths are the shortest and red are the longest.

Purple and the Visible Spectrum

Notice how the visible spectrum (defined by individual wavelengths) spans from purplish blue to red, but light can come in more colors (magenta, purple, white, etc).

diagram showing blue and red making magenta and purple, and all colors on the visable spectrum mixing to make white

This is because colors like magenta, purple, and white are made by different wavelengths of light mixed together. White light is an even mixture of all wavelengths, magenta is an equal mixture of blue and red, and purple is also a blend of blue and red, but with a higher ratio of blue. In short, white, pink, and purple do not have their own wavelengths.

If you search online for the visible spectrum you will get many sources including purple in the visible spectrum. This is not necessarily incorrect, but it can be misleading considering purple is a wide color category. The color purple includes both colors that are on the short end of the visable spectrum, and colors that can only be made by mixing wavelengths.

Violet is often used to refer to the blue-er end of purple colors, which have their own wavelength in the visable spectrum. To visualize the color of the smallest visible wavelength, think of the blue-ish purple light emitted by a black light. A black light emits mainly UV radiation^note and a little bit of visible light on the edge of UVn which is violet. This blue-ish purple is where the short end of the visable spectrum ends. Any warmer purples are created by mixing red wavelengths with blue or violet wavelengths.

note: UV radiation is harmful, but black lights are weak and typically only comprised of the less harmful long UV waves close to visible light.