A waveguide (900, 1000,1100) comprising: an optical slab (903, 1003, 1103) having a first refractive index, n 1 > 1, wherein the optical slab (903, 1003, 1103) comprises: first and second opposing surfaces arranged in a parallel configuration; and an input (910, 1010, 1110) arranged to receive light into the optical slab (903, 1003, 1103) at an angle such that the light is guided between the first and second opposing surfaces by a series of internal reflections; an optical wedge (905, 1005, 1105) having a second refractive index, n 2, wherein the optical wedge (905, 1005, 1105) comprises first and second opposing surfaces arranged in a wedge configuration, such that there is an angle between the first surface and the second surface; wherein the optical wedge has a thick end and a thin end; wherein the first surface of the optical wedge (905, 1005, 1105) abuts the second surface of the optical slab (903, 1003, 1103) to form an interface that allows partial transmission of light guided by the optical slab (903, 1003, 1103) into the optical wedge (905, 1005, 1105) at a plurality of points along the interface such that the light is divided a plurality of times, and wherein the angle of the wedge (905, 1005, 1105) allows light received at the interface to escape through the second surface of the optical wedge (905, 1005, 1105) such that the exit pupil of the waveguide (900, 1000, 1100) is expanded by the plurality of divisions of the light; characterised in that: 1 < n 2 < n 1; and the angle between the first and second opposing surface of the optical wedge (905, 1005, 1105) changes with distance from the input (910, 1010, 1110). an optical slab (903, 1003, 1103) having a first refractive index, n 1 > 1, wherein the optical slab (903, 1003, 1103) comprises: first and second opposing surfaces arranged in a parallel configuration; and an input (910, 1010, 1110) arranged to receive light into the optical slab (903, 1003, 1103) at an angle such that the light is guided between the first and second opposing surfaces by a series of internal reflections; an optical wedge (905, 1005, 1105) having a second refractive index, n 2, wherein the optical wedge (905, 1005, 1105) comprises first and second opposing surfaces arranged in a wedge configuration, such that there is an angle between the first surface and the second surface; wherein the optical wedge has a thick end and a thin end; wherein the first surface of the optical wedge (905, 1005, 1105) abuts the second surface of the optical slab (903, 1003, 1103) to form an interface that allows partial transmission of light guided by the optical slab (903, 1003, 1103) into the optical wedge (905, 1005, 1105) at a plurality of points along the interface such that the light is divided a plurality of times, and wherein the angle of the wedge (905, 1005, 1105) allows light received at the interface to escape through the second surface of the optical wedge (905, 1005, 1105) such that the exit pupil of the waveguide (900, 1000, 1100) is expanded by the plurality of divisions of the light; characterised in that: 1 < n 2 < n 1; and the angle between the first and second opposing surface of the optical wedge (905, 1005, 1105) changes with distance from the input (910, 1010, 1110).