Atmospheric
Optics
Peeping through curtains
Part of an innocent and unlit street light glints sunlight towards the camera. Graham Finney imaged it directly and then through his mesh curtain. The result - surprises.
Images ©Graham Finney
A 2-dimensional grating like the mesh curtain produces a square array of spectral orders.
3-dimensional gratings like crystals produce complex diffraction patterns that can be unravelled to reveal the crystal's structure.
Graham Finney imaged the diffraction by the mesh of a laser beam. No spectra, monochromatic spots.
Diffraction by the mesh enlarged.
Unless the mesh is tiny, the diffraction pattern is small and needs to be imaged from a distance with a telephoto lens.
There are many directions where waves from the holes of a 1-dimensional grating overlap in-phase.
The higher order overlaps are at increasing angles from the incoming beam.
Higher orders are more sensitive to wavelength and spread the spectrum wider.
The higher order glints from the street lamp above have widely spread colours.
The bright glint is surrounded by a square array of spectra. Their colours spread more and more with distance from the glint.
The mesh curtain - page background - acts as a diffraction grating.
Light leaving each hole spreads out in spherical waves. These interact with those from nearby holes. In certain directions the overlapping wave crests
are in phase and there is brightness at that angle. The condition is wavelength dependent and so with white light is split into colours and a spectrum is produced.
