# Observing rainbows

Although most of our work in Astronomy 110 Lab is related to astronomical phenomena observed when the sky is dark, there are two exceptions. One is obvious: we will try to observe the Sun, which is the star closest to us. The other is perhaps a bit less obvious: we want to observe rainbows, which are the spectrum of the Sun produced by the refraction of solar light within raindrops. Rainbows are quite frequent in Honolulu, so we can try to collect empirical data which we can later use to understand the basic geometry of this natural "light show".

So whenever you see a rainbow, please stop for a few minutes, get your log book if available, or at least get paper and pen, write down date, time, and location, observe carefully, and try to answer the following questions.

• Position and visibility of the Sun

As you face the rainbow, can you see the Sun, or is it behind you? Try to estimate how high is the Sun over the horizon, in degrees.

• Number of rainbows, their separation, and color distribution

If the primary rainbow is sufficiently strong, you will probably see a second one. Write down how many you see. Try to estimate the angular separation between them, in degrees. How are the colors distributed? Is the color distribution the same in both rainbows?

• Position, shape and size of rainbow

Does the rainbow show a circular shape? Let us assume that you can see a whole rainbow, that is to say one that reaches the ground on both sides. Try to estimate the orientation of the center of symmetry of the rainbow relative to the orientation of the Sun. Express it in degrees. For example, if the center of symmetry is exactly opposite to the Sun, then the angle is 180 degrees. Or if it is at right angles, then 90 degrees. Now consider what section of the circumference where the rainbow lies is illuminated. How long is the rainbow arc? Is it a semicircle or is it less than 180 degrees? Try to measure how many degrees. Try to imagine where is the center of the circumference partly defined by the rainbow, and estimate the angular distance between the center and the rainbow, again in degrees.

### Making sense out of it

If we are lucky, by the end of the semester we will have collected a variety of rainbow observations. Then we will devote part of one lecture to compare all these rainbow observations and explain the measured properties and the correlations between them.

### WEB RESOURCES

If you are too impatient to wait, you can visit the following website, which brings a clear explanation: About rainbows.

Roberto H. Méndez (mendez@ifa.hawaii.edu)