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90°, when Sin(B) ≈ 1): when Sin(B) > 1/D, we have D×Sin(B) > 1, thus, Sin(R) > 1, which is impossible, because Sin cannot exceed 1. Therefore, such photons are reflected back under the same angle again and again, and that is how they can be trapped inside a bead of slow time: Time Matters, Ch. 13