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

I was just looking out of window at night when I saw a tower with a light on top. It had a red light.

When I looked at it through my curtains with net on, I saw an interference fringe, one is the main light itself and band of lights on either side of it (like interference of waves).

Although there was no screen, why did I see it? Did the air act like screen, so I saw it? Was it because nets of curtain acted like slits, which produced that pattern? Or is it some other simple diffraction or anything of light? I don't think it is because of other simple reason, because interference pattern was clear.

Any process that can explain this phenomenon?

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).

• I'll second this. I have observed the same thing on our curtains. They act as a grate. It's easy to reproduce if you point a laser at it. You'll see a beautiful interference pattern on the wall. (You can even calculate the size of the grate from it!) – WIMP Dec 16 '17 at 7:30
• but what was acting as screen for the interference pattern to be seen? because the tower i saw was few km away , and the red light was on top. – hippozhipos Dec 16 '17 at 9:24
• @hippozhipos Your retinas are the screen for the interference pattern. – Mark H Dec 17 '17 at 2:23

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

• +1 for comprehensible interference picture in ASCII art. – fraxinus Dec 18 '19 at 11:13

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