An apparatus for determining a property of a downhole fluid , comprising: a first transparent member having a first side having a first planar surface , a second side having a second planar surface parallel to the first planar surface , and an angular section of the second side having an angled surface at a selected angle to the second planar surface , wherein the angled surface and the second planar surface are configured to be in contact with a downhole fluid ; and wherein the transparent member includes an aperture along an interface between the angular section and the second planar surface ; a second transparent member having a first planar side facing the second side of the first transparent member and a second planar side opposite the first planar side , wherein a downhole fluid passes through a space defined between the first and second transparent members ; a point light source at the first side of the first transparent member configured to direct light through the aperture onto the angled surface and the second planar surface ; a first detector configured to detect light exiting from the first transparent member via the angled surface at a refractive angle and passing through the downhole fluid and the second transparent member ; a second detector configured to receive light from the point light source that is reflected back from an interface between the second planar surface and the fluid at a reflective angle; and a controller configured to determine the refractive index of the downhole fluid from at least one of the refractive angle at the angled surface and the reflective angle at the second planar surface . a first transparent member having a first side having a first planar surface , a second side having a second planar surface parallel to the first planar surface , and an angular section of the second side having an angled surface at a selected angle to the second planar surface , wherein the angled surface and the second planar surface are configured to be in contact with a downhole fluid ; and wherein the transparent member includes an aperture along an interface between the angular section and the second planar surface ; a second transparent member having a first planar side facing the second side of the first transparent member and a second planar side opposite the first planar side , wherein a downhole fluid passes through a space defined between the first and second transparent members ; a point light source at the first side of the first transparent member configured to direct light through the aperture onto the angled surface and the second planar surface ; a first detector configured to detect light exiting from the first transparent member via the angled surface at a refractive angle and passing through the downhole fluid and the second transparent member ; a second detector configured to receive light from the point light source that is reflected back from an interface between the second planar surface and the fluid at a reflective angle; and a controller configured to determine the refractive index of the downhole fluid from at least one of the refractive angle at the angled surface and the reflective angle at the second planar surface . <EOS>