As that which makes visible the visible, light is the ur-medium of art. How then to mount an exhibition that takes this ur-medium as its subject, when almost any work could be included on the grounds that light is needed to reveal it? Kerryn Greenberg and Matthew Watts, the UK-based curators of Light: Works from Tate’s Collection, have responded to this challenge persuasively by focusing on two preoccupations which have endured and evolved in Western art since the late eighteenth century. Across a group of seventy works, divided into nine thematic sections, they track changes in light’s standing as an object of artistic scrutiny and shifts in the way it has been harnessed as creative material.