I noticed the above photo on a website (https://www.aapt.org/Programs/contests/pc09full.cfm?Meeting=SM09&Category=Natural&Placing=44). How the pattern can be explained bothers me for some time. As claimed in the description of the photo, the door opening has a size of 1 cm, which is far larger than optical wavelength. Then why do we see the pattern? This photo is from internet, but the phenomenon is true. I saw similar patterns myself.
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... which is far larger than optical wavelength.
We don't know what is outside the door.
I assume that the effect that can be seen has nothing to do with refraction or similar effects but that there is some object outside the door that causes this effect.
A fence or even a thin curtain could cause the effect we see here.
Note that the opened door works like a camera obscura:
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$\begingroup$ Honestly I'm not convinced by this explanation... I observed such patterns from door opening outside my office, where I don't have pillars or fences, even no curtains. Still the pattern looked pretty regular, not like from light scattered by randomly positioned tables, chairs, cabinets etc. in th room. $\endgroup$– Pu ZhangCommented Jan 14, 2019 at 7:51
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1$\begingroup$ @PuZhang However the description of the image in the link also cannot be true: Effects caused by refraction will split "white" light into different colors because different colors have different wave lengths. The effect we see here however keeps the light "white" which means that it must be some effect that can't be explained by light being a wave. $\endgroup$ Commented Jan 14, 2019 at 7:58
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1$\begingroup$ I totally agree with you that the explanation given on that webpage is suspicious and tend to believe your explanation. But now I'm still confused why the pattern looks so regular. $\endgroup$– Pu ZhangCommented Jan 14, 2019 at 8:45