An incoherent led source is directly against the fibre and covers the entire core area at the beginning of a step-profile multimode fibre and excites equally all possible forward propagating bound, refracting and tunnelling ray path directions within the core. the core index is 1.475, the cladding index is 1.465 and the core radius is arbitrary. by considering the value of the complementary critical angle, determine the percentage of the total power entering the core from the led that excites all bound ray directions. [hint: the solid angle subtended by a cone of semi-vertical angle w is 2p(1-cosw)]