Recipe for Rainbows

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Who doesn’t love a rainbow?! They conjure fanciful daydreams of leprechauns, pots of gold, unicorns, and iconic characters of youth.  Rainbows make my day, and I’ve tried to capture them on film during my many adventures over the past years.  But beyond all the warm fuzzy feelings that rainbows induce, there is some seriously cool science behind them. Let’s explore these scientific ingredients in our recipe for rainbows: refraction, reflection, and dispersion.  

Before we get into the science, we need to establish the conditions in which we see rainbows. Like any good recipe, this is our prep time!

Preparing for rainbows:

The best time to see rainbows is early in the morning or late afternoon when the sun is behind you. This diagram by Descartes illustrates proper rainbow viewing techniques.


Now to the science!

Ingredient 1: Refraction

Refraction bends the path of light waves as they pass from one material into another.  In the case of rainbows, light waves are passing into water droplets or vapor, known as rain or mist. This movement between materials causes a change in how quickly the light wave is moving, and actually bends the path of light waves.  

Ingredient 2: Reflection

Reflection occurs when a light wave bounces off a boundary.  After light enters the LaurenH_0000001raindrop through refraction, it bounces off the back (internal) side of the raindrop, exiting the other side. As the light moves from the water droplet back into the atmosphere, it once again undergoes refraction.  

So how does all this create a rainbow? We bring it together with Ingredient 3: Dispersion.

As visible light enters the raindrop, it separates into its seven color components we know as the rainbow: red, orange, yellow, green, blue, indigo, and violet. This separation is called dispersion. The angle at which light entered the water droplet influences the angle of refraction and reflection.  To get the full spectrum of rainbow colors, 40-42 degree shift from light’s entry to exit is best. 

Throw in a little rain or mist and voila, rainbow!

Rainbows are reminders that there is wonder in the world, and that science can help us understand those wonders. I hope this recipe for rainbows proves useful on your next rainbow hunt!





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