Abstract: A magneto-optical device which is responsive to magnetic fields by means of a Tyndall Effect in a particular direction exhibited by aligned magnetic colloidal particles suspended in an illuminated liquid medium. The liquid medium is contained in a transparent vessel and has magnetic particles in a very low density, such that the liquid is substantially transparent to the naked eye. A light source is positioned to illuminate the liquid medium and a light-sensing device or photocell is positioned to receive light scattered by the magnetic particles. The light-sensing device is positioned at a predetermined angle from the paraxial direction of the light source. All colloidal solutions, when illuminated by a beam of light, exhibit the Tyndall Effect, i.e. a visible scattering of light by the colloidal particles.