The End of the Rainbow

There is an interesting discussion about rainbows going on over that the Orange County Register (a former student of mine sent this to me and asked my opinion) involving a picture that some think shows the end of a rainbow. Some are questioning if the image is photoshopped. You can see the picture in question on the right.

Okay, rainbows do not have an end. If you have ever been in a plane, you can see rainbows form complete circles! I have seen this and it is pretty cool. What they mean obviously is the point where the rainbow hits the ground. To me, I see what they call the "end" of a rainbow all the time. Sometimes it appears over a mountain, others over the ocean, sometimes over a house.

Let's back up. In order to get a rainbow, the Sun must be low in the sky. There must be rain on the opposite side of the sky from the Sun from where you are standing. Sunlight passes through the raindrops and reflects off the back. However, as sunlight passes through the raindrops, it also bends due to a phenomena called refraction. Blue light refracts more than red light. Therefore, when the reflected light heads back to you, it has been split up into different colors. You can see the path light takes through a raindrop in the image on the left and notice how the colors separate.

You can fine the center of the circular rainbow by drawing a line from the Sun and through yourself and extend it away on the other side. This will show you the center of the circle somewhere underground (except for rare cases when you see a rainbow right after sunset). The circle forms a cone with you at the tip an an angle of 42 degrees to the rainbow.

So let's go back to the photo. It was taken on highway 241 North in California at 4:59pm on February 7th. The GPS cooridnates are given on the OCR site. I popped them into my computer and found the Sun was 15 degrees high in the sky at an azimuth of 239 degrees. The center of the rainbow is at 59 degrees putting the left side of the rainbow at about 17 degrees (slightly east of north) which jibes well with someone driving north on 241 seeing this site. Since the Sun is low, we see almost half the circle and this rainbow has a nice steep arc which also agrees well with the time and geometry of the situation.

Some in the discussion argue this is a "sparybow" created by spray kicked up by the car tire (which also gives the nice illusion of the end of the rainbow). It does appear the tires are kicking up spray that makes the bottom of the rainbow brighter. However, the person who took the picture says they say an entire arc of the rainbow. That single car could not kick up enough spry to create a rainbow that extends that high in the sky! I suggest you could call it a hybrid...the bottom part may be a "spraybow" but you definitely have a rainbow there as well.

So what do the people in that car see? They see a rainbow in front of them! As you move toward a rainbow, it continually moves away from you. You can never reach the true end of a rainbow no matter how hard you try. In fact, everyone sees their own personal rainbow. The light I see entering my eye comes from a different set of raindrops than a person standing next to me becuase the angles between the person and the Sun change!

So enjoy the rainbows...but don't go chasing that pot of gold!

Reprinted with permission from the Half-Astrophysicst Blog.