# Strange interference pattern of light on top of tower, pattern was seen on air. What was it?

What you are seeing is an interference pattern, similar to double slits or diffraction gratings. You can confirm this comparing the light pattern when you are looking straight through the curtain (when the curtain is perpendicular to the line from you to the light) and when the curtain is at an angle. Angling the curtain makes the threads appear closer together, so the interference fringes will spread out more.

For the curious, here's what a far off stop light looks like through motel curtains:

The pattern is easier to see when the source in monochromatic (LEDs and the like).

The threads in the curtain are acting like a grid that bends the light. The finer the fabric of the curtain, the more the light is bent.

Now you wonder why you see the resulting interference pattern even though there is no screen behind the curtain. The reason is, that your eyes detect the direction from which the light is coming.

When the curtain bends the light, the almost uni-directional light from the distant source changes direction. Everywhere that it hits the curtain. So, while you have a uni-direction, flat electromagnetic wave before the curtain, you have a number of flat electromagnetic waves behind it, each with its own direction of propagation.

Your eye detects each of these flat waves as coming from a single point-like, far away source. It locates the source of the unbent wave where the actual light source is, but each of the other waves signals a different light source location due to its direction of propagation, yielding its own image on your retina.

Of course, the bent light that you see has passed the curtain at a different location from where the unbent wave has passed it. As such, if you have an object right in front or behind the curtain, that object can block some of the light source's images.

light
image         |                    ___>                ___>
<....         |               ____/               ____/
.....    |          ____/               ____/
....|     ____/               ____/
______________|____/___________________/__________________>
|    \____      ____/
|         \____/
|     ____/    \____      Your
______________|____/______________\___> (o >
distant       |    \____      ____/     eye
light         |         \____/
<------       |     ____/    \____
______________|____/______________\_______________________>
....|    \____               \____
.....    |         \____               \____
<....         |              \____               \____
light         |                   \___>               \___>
image      curtain


Regarding the "no screen" part: Your retina can be a pretty good "screen" in this case.