The principle of Reflex Level Gauges is based on the difference in the refractive indices of liquid and vapour. The liquid column is contained within the recess of the liquid chamber behind the sight glass which is clamped to the gauge body. The sight glass has prismatic right angled grooves on the side facing the liquid and vapor space. Light rays entering from outside the gauge are either absorbed or reflected depending upon whether they enter the liquid or vapour space. When the ray of light encounters the surface of one of the grooves in the vapor space, it is reflected to the opposite surface of the grooves and from there, totally reflected back to the direction of observation. Thus vapour
space appears as silvery white. When the light ray encounters the surface of the grooves in the liquid space, it is totally absorbed thereby the liquid behind the glass appear black.