The figure below depicts a simplistic optical fiber: a plastic core (n1 = 1.70) is surrounded by a plastic sheath (n2 = 1.64). A light ray is incident on one end of the fiber at angle ?. The ray is to undergo total internal reflection at point A, where it encounters the core-sheath boundary. (Thus there is no loss of light through that boundary.) What is the maximum value of ? that allows total internal reflection at A?