For the boundary between the glass and crown water, the critical angle is 61.0 degrees. For the boundary between glass and crown water, the critical angle is 61.0 degrees. Make particular note that the critical angle is an angle of incidence value. For the water-to-air limit, the critical angle is 48.6 degrees. So the critical angle is defined as the angle of incidence that provides a 90 degree angle of refraction. For the water-air limit, the critical angle is 48.6 degrees. Critical angle = Θi that gives a Θr value of 90-degrees According to Snell’s Law equation, a generic equation for finding the critical angle can be derived easily. Note that the critical angle is an angle of incidence value. The critical angle can be calculated from Snell's law by setting the refraction angle equal to 90°. Get … α c = sin -1 (n 2 / n 1) Where Ac is the critical angle. For the water-air boundary, the critical angle is 48.6-degrees. The following equation is used to calculate the critical angle through two mediums. The actual value of the critical angle is dependent upon the combination of materials present on each side of the boundary. Condition for total reflection For total internal reflection, Incident angle > Critical angle Medium where light coming form must have higher index of refraction compared to second medium (n The actual value of the critical angle is depending on the combination of materials present on each side of the boundary. Θcrit= sine-1 (nr/ni) For the crown glass-water boundary, the critical angle is 61.0-degrees. So the critical angle is defined as the angle of incidence that provides an angle of refraction of 90-degrees. n1 is the refractive index in medium 1. n2 is the refractive index in medium 2. Critical Angle Formula.