the rock kind not the lamp
Snell's Law: n1*sin(theta1) = n2*sin(theta2), where n1 = refractive index of 1st medium theta1 = incident angle inside 1st medium n2 = refractive index of 2nd medium theta2 = refracted angle inside 2nd medium At the critical angle, the refracted angle inside the 2nd medium is 90 degrees, and the sine of 90 = 1. The equation reduces to: n1*sin(theta1) = n2 Solving, sin(theta1) = n2/n1 To get the angle, we use arcsin: theta1 = arcsin(n2/n1) Now we need the refractive index of water and fused quartz. Water is 1.33, fused quartz is ~1.46, so theta1 = arcsin(n2/n1) = arcsin(1.33/1.46) = 65.64 degrees. Note that this only works for light rays going through the fused quartz and striking the water interface; light rays going through water striking fused quartz bend towards the surface normal (since quartz has a higher n) and therefore do not have a critical angle.
