by Chris Spruck
Because of my background in science, each time I’m asked to write for Perspective Online, I can’t help but think of geometry and things like parallax angles for measuring the distance of astronomical objects, optical illusions, solar and lunar eclipses, horizon lines and depth of field when composing a photograph, and other things with “perspective.”
I happened to take this photo of a rainbow from my office view, as I watched a recent rainstorm recede to the east towards the end of the day. Angles have to be just right to see a rainbow, and as conditions lasted just a few minutes, I was lucky to catch this one.
Rainbows always form directly opposite the sun and are formed by the refraction and reflection of sunlight through water in the atmosphere. Refraction is simply how light changes direction slightly as it passes from one medium to another – in this case, from air to water droplets in the air. Light enters a drop of water, which acts as a prism, separating light into its component colors, then is reflected off the back of the water drop (“back” being relative to your perspective). The light is refracted again as it leaves the water drop and returns to your view. Refraction is easy to see without needing a rainbow. Hold a glass of water close to your eyes, look around the room, and notice the distortion of objects. You might be surprised to see that text is mirrored.
You can only see a rainbow at about 42 degrees from the direction opposite the sun, as they are a bit of an optical illusion and sometimes can appear quite far away or quite close. It’s all due to that perspective thing. All light passing through water droplets is refracted and reflected (and some passes right through the drops), but the individual colors leave the water droplet at slightly different angles, so you really only see one color from a given raindrop. Lower or higher drops (there’s perspective again) return different colors of light to your eye, thus you see the typical bands of a rainbow.
To see the rainbow in my photo at that location, enough water droplets had to be present but not dense enough to block light like thicker fog could. The angle of the sun had to be less than 42 degrees above the horizon, and it was gradually getting lower behind my building, so in just a few minutes the light level changed enough to where the rainbow faded away before my eyes. This was definitely a case of having the right perspective!
Chris Spruck is a web applications developer for Distance Education
