# Time for an Interesting Photo: In-Flight

To all,

Here is an interesting photo I captured while in flight. I love looking out of the window during flights because you can see a lot of very interesting things, and this one I had not seen before. It it the shadow of the airplane on an adjacent cloud. This is neat in itself, but not very curious. The curious part is the ring of color bands like a rainbow surrounding the shadow. As a note to those concerned, my phone was on airplane mode, so no interference created here. You’ve got to obey those flight attendants!

A note also to any skeptics, I have not altered this photo in any way other than cropping. This is merely what the camera on my phone captured. Here a two more pictures to confirm it was not a fluke. I did not crop these photos, so the shadow appears a little further away.

In the photos the color rings where always with the shadow and the shadow appears “on top” of the rings. Also, the rings only show when projected on the cloud. With these things in mind, I conclude that the color rings are not an effect created by the camera or with my window, but the color bands are actually on the nearby clouds.

Lets try and understand why these rings of light were created.

My first thought brought me to the diffraction or bending of light as it passes around an object. As an example, the Greeks made their columns a little wider at the center so that they would appear of constant diameter. This is because as light diffracts around the column it is pulled inward making them look a little smaller at the center, see photos below. In the photo on the right, the light diffracts around the window frame making it look like there is a hole. But this has nothing to do with a rainbow ring, so we need more information.

My next thought brought me back to introductory physics class and when we learned that light is both a particle and a wave. The canonical experiment is single or multiple slit diffraction experiments. As light passed through a very narrow slit, it would bend and create this line of alternating colors like a rainbow.  When it passed through a hole it would make rings like we see here. This is good, we are making progress. (For those who did not take that introductory physics class, I am sorry, but I don’t really have the space here to get you up to speed.)

Two things are left unexplained. The plane is not a hole, but a dot (compared to the sun), and the plane is not round, but plane shaped. If I remember correctly, diffraction happens just the same with a dot instead of a hole. This makes sense because the light will still bend. But why did the weird shape of the airplane make perfect circles of light.

I am still a little perplexed. Can any readers help fill in the holes of understanding regarding this interesting photo? Is my thinking so far correct or is something totally different at play here?

Thanks,
Ben Washington

### 7 thoughts on “Time for an Interesting Photo: In-Flight”

1. Cirrus clouds are associated with a number of optical effects including: glories, halos, sundogs and fire rainbows. A glory appears as concentric rings that appear around the shadow of the observer. Glories appear in Cirrus clouds when the ice crystals are aspherical, and between 0.009 and 0.015 millimeters.
You are kind of on the right path with thinking about how light passes through objects. Although the phenomenon is not fully understood. It is thought to be produced by the interference caused by light rays entering the droplets at diametrically opposite points.

• I checked out the sundogs and fire rainbows, fascinating and beautiful. Thanks also for the insight. I think it is very interesting with all the learning going on in the world, something like this is still not fully understood. Perhaps, not a whole lot of research is going into this though.

2. I think I might be able to help you here. My name is Mike and I’m a pilot based in London.

Don’t worry – you’re not insane. This is a pretty common (and cool) phenomenon. You’re bang-on with thinking you’re seeing diffraction at work here. The light bends slightly when passing the aircraft and hence you get the circular rainbow effect with the plane’s shadow in the middle. It only really shows up on cloud because it’s actually very faint, and you need strong light onto fairly dense cloud to really see it well.

The reason it’s circular is actually slightly counter-intuitive. The shape of the ‘projection’ is actually determined by the shape of the source, and not the shape of the ‘hole’ or ‘dot’. What you’re actually looking at is a projection of the sun onto the cloud, and the interference (rainbow) is caused by the bending of the light. If this is difficult to picture you can perform a simple experiment. Take a piece of card and cut out a shape (a star, square, little man with a dog.. anything) then, in a dark room, shine a light through it and onto a wall. You’ll notice that when the card is close to the wall, you can see the shape you cut out, but as the card is moved closer to the light source it gets fuzzy – more like the shape of the light source. This trick is used to watch solar eclipses without having to look at the sun, and indeed, cameras take advantage of this to fit images onto the small film.

Anyway, that’s pretty much what you’re seeing. If you want to know what’s cooler – that’s seeing it from the front – the ‘shadow-plane’ collides head on with you as you fly into the cloud.

Ok, next question for the class… as I sit at the front of the aircraft, it doesn’t matter if I move my head left or right, up or down, the ‘shadow-aircraft’ will always fly directly at my head. From the Captain’s seat, (s)he will also see the ‘shadow-aircraft’ fly directly at his/her head. Why is this?

• Thanks for the insight. I have seen what you are talking about with a spotlight. That does help explain why the rings are circular. From other peoples’ comments, there does seem to be some mystery remaining. That is a really neat photo you took, thanks for sharing.

Regarding you question, is it just because at the plane gets closer to the cloud the shadow get closer as well. Maybe I am not understanding what is happening exactly. Can someone else help again?