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120° Parhelia

Upper image by Robert Goren in Oregon

Lower seen by Mike Rubin in Jerusalem during the great March '07 halo display.

Colourful sundogs, 22° parhelia, a hand's span from the sun are familiar. Less so are the white patches much further along the parhelic circle. 120° parhelia can, as here, be extremely bright but very often they are overlooked because they have no colours to distinguish them from clouds.

They are formed by plate crystals. A ray can enter the top face, reflect internally from two side faces and finally leave through the lower face - path 1,3,8,2. The overall light deflection is always 120° regardless of the crystal orientation. The colour dispersions at ray entry and exit are in opposite directions and cancel to yield a white halo. The ray's route needs several optically perfect faces, freedom from internal defects and thick plates. Rare.

Are the 120° parhelia always lacking in colour?   See the next OpticsPOD.

Images ©Robert Goren & Mike Rubin

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