More Dust

OPOD continues its previous (1,2) dusty theme.   

Not volcanic dust this time but ordinary household dust on windows.

Monika Landy-Gyebnar was decorating her Hungarian apartment with her husband and cat when she noticed Quételet rings across the reflection of the light bulb in the double glazed window.

The cat is curious about them. His name is Shaman and I’m told that he is an avid optics fan. The wire netting stops him jumping out of the window to search for circumscribed halos.

The multi-coloured fringes, “Quételet rings” can be seen on dusty windows and mirrors and outdoors on calm water surfaces laden with pollen, dust and even algae.

They are generated when small micron-sized particles rest on or are close to a reflecting surface.

Light reaches the particle along two routes (1) directly and (2) after being first reflected from the surface. The particle scatters the incident light waves predominantly forwards into a series of outgoing spherical waves.

The two sets of outgoing waves overlap and combine. In directions where the overlapping wavefronts have the same amplitude direction (in phase) they reinforce and there is light. In some other directions the waves are out of phase and they cancel each other out.

The cancellation or interference condition is wavelength dependent and so the resultant fringes are coloured.

Images ©Monika Landy-Gyebnar, shown with permission.
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