Criteria for Negative Refraction in Active and Passive Media P. Kinsler, M. W. McCall Blackett Laboratory Imperial College London, Prince Consort Road, London SW7 2AZ United Kingdom. Microwave Opt. Tech. Lett. 50, 1804 (2008). We review three commonly used conditions that characterize negative refraction in isotropic dielectric-mangetic materials. We show that whilst all the different conditions are equivalent for passive media, they are distinct for different types of active media.