Advice directly from the Annals of The Royal College of Surgeons of England,
Immediately before wearing a face mask, wash the spectacles with soapy water and shake off the excess. Then, let the spectacles air dry or gently dry off the lenses with a soft tissue before putting them back on. Now the spectacle lenses should not mist up when the face mask is worn.
The face mask directs much of the exhaled air upwards where it gets into contact with the spectacle lenses. The misting occurs from the warm water vapour content condensing on the cooler surface of the lens, and forming tiny droplets that scatter the light and reduce the ability of the lens to transmit contrast.1 The droplets form because of the inherent surface tension between the water molecules. Washing the spectacles with soapy water leaves behind a thin surfactant film that reduces this surface tension and causes the water molecules to spread out evenly into a transparent layer. This ‘surfactant effect’ is widely utilised to prevent misting of surfaces in many everyday situations.