As spring turns into summer and we spend more and more time outside, we joyfully reach for our sunglasses – but have you ever wondered how polarising filters work? Polarisation is a property of light, and refers to the orientation of the light’s electromagnetic wave. The electric component of this wave oscillates at right angles to the light’s direction of travel, and in normal light it is random, with all orientations exhibited. In polarised light, all the light’s waves oscillate in the same plane. Light becomes polarised when it is reflected, and partially polarised by its passage through the air. Polarised sunglasses are designed to block the polarised reflected light, hence reducing glare from the ground and water. Other materials can also alter the polarisation of light, so studying materials in polarised light can reveal, for instance, stresses in plastic, growth patterns in crystals, and the fine structure of transparent living organisms.