“When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return.” –Unknown
Someone showed me a picture yesterday, and my initial reaction was simply, “WOW!” See for yourself:
But, as a physicist, I look at something like this and I immediately need to figure out what’s going on here.
First off, you notice that this is clearly some type of fighter jet, being photographed from below with the Sun illuminating it from above. A little google-fu reveals that the silhouette matches that of an F-22 Raptor.
This plane is easily capable of supersonic speeds, and I wouldn’t be surprised if its speed wasn’t exceeding (or at least approaching) Mach 1. The fact that we’re moving somewhere around the speed of sound is important, and we’ll need to remember it for later.
But the star of the top picture is clearly the rainbow trail behind it. Most of you have seen such a rainbow trail before, similar to when you have the Sun shining on a film of soap or oil in a puddle on the street.
But the thin-film rainbow, above, caused by light refracting into the oil, reflecting off of the water beneath, and then making it to your eye, likely doesn’t have anything to do with the one we’re seeing. The closest thing we have to what’s going on, coming in at number 8 on a list of top 10 strangest natural phenomena, is the Fire Rainbow.
The Fire Rainbow happens when you have high-altitude sunlight (58 degrees or higher) hitting special types of ice crystals known as hexagonal plate crystals, found naturally in very-high altitude cirrus clouds.
The light enters through the top face of a horizontally-oriented hexagonal crystal and leaves, after some internal reflection(s), through the bottom face. It produces a spectrum of colors, visible only to an observer who sees the ice crystals at an angle of 46 degrees.
But what’s going on with the plane is an incredibly special example of this.
Now, did you remember that this fighter plane was moving close to the speed of sound? When an aircraft approaches the speed of sound (Mach 1), the air pressure above the wing becomes very, very tiny, and the airflow behind the airplane becomes very turbulent, and filled with vortices.
This is in contrast to the air elsewhere around the plane, which is at a relatively normal temperature and pressure. When you have two regions of air touching each other, one with normal temperature and pressure and one with practically no air in there, something really interesting happens. The air rushes into the region with practically no air, and because it’s practically a vacuum, it cools tremendously! This ultra-fast cooling is so powerful that it can not only pull water vapor out of the air and turn it directly into liquid water,
but it can immediately turn that liquid water into the special form of — you guessed it — hexagonal ice crystals!
And the turbulent motion of the airflow combined with these oddly-shaped crystals, with the Sun at just the right angle, combines to produce this spectacular rainbow effect! And that’s the physics of where this rainbow comes from!