I saw something quite odd this morning and I can’t quite explain it. When I woke up, there was what appeared to be a diffraction pattern projected on my wall, resulting from a small gap between the curtains (a single-slit diffraction pattern). But as far as I understand, single slit diffraction only occurs when the slit width is roughly the same order of magnitude as the wavelength of light.
In addition to that:
- the light source is reflected sunlight (which should consist of lots of different wavelengths) from the building opposite
- it was hard to see a whether the different bands had different intensities
- there were two bands which were red (why?)
- too many bands to be a diffraction pattern? Given the wall is probably ~5m from the window, the angle between each “slit” is actually tiny.
Here’s the curtain & slit:
If anyone could shed any light on this, it’d be appreciated 
#1 by Jacob at September 13th, 2009
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You raise an interesting point. But im unsure whether it was a diffraction pattern or not as like you said it would require the slit to be roughly the magnitude of the wavelength. But also because the source would need to be roughly coherent and it would be white light coming through. Having a look at the picture of your curtains my guess would be that it was light coming through the material of the curtains itself and the dark/light fringes were caused from crumples in the curtains?