a < \frac{λ_{0}}{n_{F} \cos α_{c r}} = \frac{1, 25 \cdot 10^{- 6}}{1, 465 \times \cos (1, 488)} = 10^{- 5} m = 10 μ m

${a < \frac{\lambda_0}{n_F \cos \alpha_{cr}} =\frac{1,25 \cdot 10^{-6}}{1,465 \times \cos⁡(1,488)}= 10^{-5}\, \mathrm{m}=10\, \mathrm{\mu m}}$

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