Find A Rainbow Day

What the day is

Find a Rainbow Day is observed on April 3, early in the season that supplies the right combination of sun and passing rain.

Origin and background

A rainbow is not an object, and it is not anywhere. Sunlight entering a raindrop is bent on the way in, reflected once off the inside of the back of the drop, and bent again on the way out, leaving at a consistent angle of about 42 degrees from the direction it arrived. Different wavelengths bend by slightly different amounts, so that angle varies by a degree or so across the colors, and the light separates into a band.

Three consequences follow, and between them they account for everything odd about rainbows. The first is that a rainbow always appears directly opposite the sun, centered on the shadow of your own head, which is why you will never see one while facing the sun. The second is that you cannot approach it, because it is defined by an angle from your eye rather than by a location, so it moves exactly as you move. The third is that no two people ever see the same rainbow, since the light reaching your eye comes from different raindrops than the light reaching the person standing next to you.

The geometry also imposes a timetable. Because the arc sits 42 degrees from the point opposite the sun, a sun higher than 42 degrees in the sky pushes the whole bow below the horizon. Rainbows around midday in summer are therefore impossible, and the good ones come early and late in the day.

A rainbow is actually a full circle, and the ground is simply in the way. From an aircraft the complete ring is visible.

The fainter second bow has its colors reversed, the result of one extra reflection inside the drop, and the sky between the two bows is noticeably darker.

How to take part

• Look opposite the sun, low in the sky, early or late in the day.
• Turn your back to the sun and find your own shadow first.
• Look for the darker band of sky between a double bow.
• Make one yourself with a hose on a sunny afternoon.