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Grades K–2 · Earth & Space

How do water drops make a rainbow?

A rainbow can appear when sunshine reaches rain in front of you. The Sun is behind you, even though the colors are ahead. Tiny drops are turning some sunlight back toward your eyes.

Sunlight looks almost white, but it contains many colors together. When light enters a water drop, its path bends. Some light then bounces from the inside back of the drop. When it leaves the water, it bends again.

The colors bend by slightly different amounts, so they leave in different directions. From one drop, one color may reach your eye while other colors miss it. Other drops send other colors toward you. Many drops together make the curved band we call a rainbow.

Follow the light through one enlarged drop, then look at where the Sun, observer and rain must be for a rainbow to appear.

Rain drops redirect sunlight toward an observerAn enlarged circular drop shows physically refracted red and blue light paths with one internal reflection and different exit directions. A lower side view places the Sun behind the observer and rain ahead. One selected outgoing direction returns light to the eye; other colors from that same drop travel elsewhere.Colors already in sunlightOne drop, enlargedSunlightBendson entryReflectinsideBend againColors leave in slightlydifferent directions.Where the observer standsSunRainLight returnstoward the eye.Sun behind · rain ahead
Sunlight bends on entering a drop, partly reflects inside and bends again on leaving. Different colors leave in slightly different directions. The observer faces the drops with the Sun behind them.

The drops do not paint the sky or create new colors. They separate colors that were already in the sunlight. A rainbow also depends on where you stand: your eyes receive light from the drops in the right directions. Someone nearby has a slightly different set of drops making their rainbow.

About this explanation

Only red and blue paths are drawn inside the drop. A real rainbow contains a continuous range of colors.

Starting knowledge: Recognizes sunlight, water drops, and the idea of light changing direction.

Sources

  • NOAA NESDIS — What Causes a Rainbow?
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