The Crystal Planet: Magnesium Sulfate Birefringence
Jasmine Wu
Award: Top 100
Award: 1st Place Contrived
School: marlborough school
Teacher: lisa ellis
Category: Contrived
Photo #21116
The Crystal Planet: Magnesium Sulfate Birefringence
Photographing epsom salt birefringence, I expected orderly needles and clear glass. Instead, a planetary limb burst from the salt, painting bands of blue and orange above crystalline terrain.
I grew magnesium sulfate crystals by evaporating a supersaturated solution over 24 hours, using undissolved grains as nucleation sites. Dendritic fans sprawled across the walls as rapid supersaturation caused branching, while slower supersaturation at the bottom produced long needles. I photographed them through a DIY polariscope, a polarizer crossed at 90° to my LCD monitor's linearly polarized white light.
Magnesium sulfate is birefringent, resolving light into two orthogonal rays due to anisotropy, asymmetric molecular bonding along different axes. With refractive indices of 1.433 and 1.461 (birefringence 𝚫n=0.028), one ray travels slower through the crystal and accumulates retardation proportional to crystal thickness. At the second polarizer, only components along the polarization axis pass through, forcing the rays to recombine and interfere. The resulting color follows the Michel-Lévy chart, where 0.035mm thick magnesium sulfate displays vivid orange.
Here, vibrant colors mark thin crystals of low-order interference, while pastel colors indicate increasing thickness and high-order interference. Dark areas reveal crystals with their optical axis aligned with the LCD polarizer, where light remains unsplit and is blocked by the second polarizer. Unexpectedly, the colorful arc of normally isotropic glass shows photoelasticity, where residual manufacturing stress induces anisotropy. Physics paints birefringent crystals and maps the stress in glass. Different materials, through polarized light, create yet another mesmerizing world I never knew existed.
|
