[The (Lucky?) Thirteenth Blog] Optics.

       Since it was exceptionally rainy this week (such lovely weather we had...), I was bound to see some rainbows. I live in Manoa, though, so rain and rainbows are not that big of a deal. However, as I was coming home from school on Tuesday, through the pouring rain, I saw something I've never seen before... A DOUBLE RAINBOW! [I tried to take a picture with my phone (because of course I was thinking hmm, maybe I can use this for a physics blog), but it didn't turn out so nice... Eh. Well you can sort of see the second rainbow right above the tree if you look reeeeeally hard (unless I'm just imagining things. O.O Highly likely.)]

But anyway, since my picture was not very clear, here is a better one:

       And now I will attempt to explain how this double rainbow phenomenon comes to be. Being able to see a rainbow all depends on where you are relative to rain and the sun. Rain must be going away from the sun and the sun must be shining from behind you. The formation of the rainbow itself is due to two processes: refraction and reflection. When the white light (a mixture of all colors) from the sun comes into contact with a raindrop, the light bends and breaks into its constituent colors (dispersion) based on their wavelengths. This process of light bending as it enters a new medium (air --> water) is called refraction.  Red, with the longest wavelength, refracts least, while violet, with the shortest wavelength, refracts most. The light then continues to travel in a straight line until it reaches the inside surface of the raindrop, at which point it internally reflects. After being reflected, the light refracts again when it leaves the raindrop and enters back into the air. 


       However, the array of colors that we see does not come from a single raindrop. From some we see the red, from others we see the orange, the yellow, and so forth. This is because the angle at which the light refracts  in a raindrop only allows us to see a single color from that particular one - colors refracted above would be above our eyes, and colors refracted below would be below them. Now that is how the first rainbow is...born...but what about the second? Well, the secondary rainbow is the result of two internal reflections. Instead of reflecting once then exiting the raindrop, light reflects a first time and then a second before it leaves. Because some light is lost during each reflection (since it is not totally internally reflected), the secondary rainbow appears more faint than the primary. The second reflection also reverses the order of the colors, resulting in a rainbow with violet on the outside and red on the inside. Lastly, the rainbow is curved as a semicircle because the set of raindrops that have the right angle between us, the sun, and the raindrops lies on a cone in which we, the observers, are looking from the point directed toward the sun. Of course we only see one half of it because the ground gets in the way, so that's why it looks like a semicircle. Phew. Who knew that this seemingly magical phenomenon was simply a matter of physics! :]