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Light from free space is incident at an angle $\theta _{i}$ to the normal of the facet of a step-index large core optical fibre. The core and cladding refractive indices are $n _{1}=1.5$ and $n _{2}=1.4$, respectively.

The maximum value of $\theta _{i}$ (in degrees) for which the incident light will be guided in the core of the fibre is _________