WAEC Physics 2011 Paper

Concept:Total internal reflection (TIR) occurs when light travels from a denser medium into a rarer medium and strikes the boundary at an angle greater than the critical angle.Explanation:Inside an optical prism, glass is denser than the surrounding air, so light moving toward a prism face travels from a denser to a rarer medium.TIR is possible only when the angle of incidence ($i$) exceeds the critical angle ($c$) for that glass-air surface.If the ray enters an optically denser medium, it bends toward the normal and TIR cannot occur, making option A incorrect.If the ray is incident normally, $i = 0^\circ$, so it passes straight through without internal reflection, making option C incorrect.Merely traveling from the prism to air is not enough; the ray must also strike the surface at $i > c$, so option D is incomplete.Therefore, the correct condition is that the rays must be incident inside the prism at an angle greater than the critical angle.Answer:B. are incident in the prism at an angle greater than the critical angle