Work out critical angle instantly with clear inputs, formula shown and shareable results.
The critical angle is where the refracted ray would graze the boundary at 90 degrees, so sin(theta c) = n2/n1. Glass to air at 1.5 and 1.0 gives 41.8 degrees - which is why a 45 degree prism reflects totally without any coating. No critical angle exists when the second medium is denser.
Critical angle
sin(theta c) = n2 / n1, requires n1 > n2
Beyond it
all light is reflected internally
About 48.6 degrees for n = 1.333, which produces Snell's window - the circular view of the sky seen from underwater.
Because light striking the core-cladding boundary beyond the critical angle is reflected with essentially no loss, guiding it for kilometres.