Double Treat

I’d been inside all day, working while listening to the rain and the occasional clap of thunder. They are rare here and so I’ve been making sure to thoroughly enjoy the parade of storms we’ve been having over the last several weeks.

Today (Tuesday 9th!) was definitely a day I’d planned to stay in, rain or not, since I wanted to dig further into a project I’m working on, and focus on it all day. And so focus I did, until late in the afternoon I noticed a lovely light on the buildings and trees, telling me that the sun had emerged from the clouds just before it dropped below the horizon – peeking through that gap between the clouds and the sea that, now I think of it, must always exist quite clearly if there is a localized set of clouds over the region, due to the storm. I thought I’d go outside for the first time of the day and look at this evening light, and I noticed it was still raining and there was a rainbow! Actually, it turned out to be two. A double treat stretching majestically across the sky. I grabbed a piece for you. [Update: Several pieces. Glued two together for you to get the full effect. See above. It was raining, so was not so careful with my alignments when snapping in my haste to escape….]

Notice that the colour sequence (ROYGBIV) runs backwards on one as compared to the other. The internal reflection that happens to light when it goes inside the raindrops, resulting in the rainbow upon emerging, can have a second bounce, resulting in a second rainbow. A bit of tracing of the rays for different colours of light shows the reversal but I’m a bit too tired to write the details down or draw a diagram (it is way past my bedtime), but I don’t have to since there is a nice Wikipedia article here, and also, an applet you can play with here.

Here’ a thing. A rainbow is not a specific glowing thing that everyone is looking up and seeing. Everyone’s rainbow is their own personal rainbow, identical to everyone else’s, sure, but their own nonetheless. The light is scattered by the raindrops (differently for each colour component of the white light from the sun), in a range of directions, but there is a specific angle at which the scattering is most intense . It is about 42 degrees (from the incident line of the sunlight coming in to hit the raindrops) for the first rainbow and 53 for the secondary one. Someone standing at a different position from will make that 42 degree (or 53 degree) angle work out by looking in a slightly different direction from you. So their rainbow is not the same as yours, in a sense. Yours is your very own! (Another bonus: You don’t have to share your pot of gold either… although you can never get to the end of the rainbow to find it… the 42 degrees can’t be maintained, right?.)

Enjoy! And keep looking up.

-cvj