Polarising filters reveal hidden properties in these crystals.
Crystals of the antiviral drug 2-3 dideoxyadenosine, also known as ddA, seen through a microscope under polarised light. The drug is closely related to AZT or azidothymidine.
Like many crystals, this one refracts light differently depending on its polarisation, or the direction in which the electromagnetic waves are waving. A polarising filter allows this behaviour to be seen.