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Find a Rainbow Day Spectroscopy

Apr 1, 2026 | Applications, Atmospheric, Color Measurement, Displays & Radiometry, News, Optical Metrology, Solar SpectroRadiometry, SpectroRadiometry

The Science Behind Rainbows

Rainbows form when sunlight interacts with water droplets in the atmosphere. As light enters a droplet, it refracts (bends) due to the change in medium from air to water, which slows down the light waves. Once inside the droplet, some of the light reflects off the inner surface and then exits the droplet, refracting again as it re-enters the air. This repeated bending and reflection cause the white sunlight to separate into its individual wavelengths, revealing the familiar sequence of colors: red, orange, yellow, green, blue, indigo, and violet.
Each color appears at a slightly different angle because different wavelengths of light bend by different amounts, a process known as dispersion. Red light, which has the longest wavelength, bends the least, while violet light, with the shortest wavelength, bends the most. This variation in bending angles creates the arc-shaped spectrum we see in the sky.

The position of a rainbow depends on the observer’s perspective and the location of the sun. Typically, rainbows appear when the sun is behind the observer and the light is refracted at an angle of about 42 degrees. If conditions are just right, a double rainbow can occur, with the second, fainter arc appearing outside the primary one due to an additional internal reflection within the raindrop.

Cite: Ask BOM: how do rainbows form?

Spectroscopy and Rainbows

The way a rainbow separates light into its individual colors is the same principle that spectroscopy relies on. Spectrometers use diffraction gratings to break light into its spectral components, allowing scientists to analyze materials based on how they absorb or emit different wavelengths.

This technique is used across various fields, from environmental analysis and chemical identification to food safety and pharmaceutical research. Just as a rainbow reveals the different colors hidden within white light, a spectrometer unveils critical information about the composition of substances.

Concave-grating spectrometer vs. Czerny-turner

Fun ways to celebrate Find a Rainbow Day

Observe a rainbow: If conditions are right, take a moment to enjoy this natural light display and consider the science behind it.

Experiment with light: Use a prism or diffraction grating to split light into its spectrum, replicating the rainbow effect indoors.

Explore spectroscopy: If you have access to a spectrometer, try analyzing different light sources and materials to see their unique spectral fingerprints.

Happy National Find a Rainbow Day!

At StellarNet, we celebrate the beauty of light and the science that helps us understand it. Whether in the sky or in the lab, rainbows remind us of the incredible insights that spectroscopy can provide. Whether it’s a photo of a rainbow or a spectrum captured with a StellarNet spectrometer, share your discoveries on social media and tag us!

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