Click photo for larger version.

Honors for Captivating Detail

Uncovering the Hidden Beauty of Crystals Under Polarised Light

Ascorbic acid, known as vitamin C, is in many foods that we consume daily, ranging from citrususes, strawberries to brocolli. In its pure form, it forms plainly looking white cristals. With an interest in microphotography, I observed ascorbic acid crystals which crystalise out of a saturated solution when thinly smeared on a glass microscope slide which I prepared. What I observed was suprising – an intricate spectrum of colours arose.

But why do crystalised solids show different colours under polarised light under an optical microscope?

When polarised light – light that vibrates in a single direction – passes through a crystal, it splits into two light rays vibrating perpendicularly to each other. As these two light rays pass through the crystal, they travel at different speeds because the crystal's refractive index (RI) varies with the direction of the light’s vibration. This is due to the anisotropic nature of the crystal, meaning it has different properties in different directions. Upon exiting the crystal, the two rays reach the upper polarizer, which recombines them. However, as the two rays travel at different speeds, they emerge slightly out of phase with each other. This phase difference causes them to interfere with each other when they recombine. The interference can either amplify or cancel out certain wavelengths of light, producing a spectrum of colors. These colors are known as interference colors. The greater the difference in RI values, the higher the birefringence, which results in more intense colours.