$\bullet$$\bullet$ Light pipe. Light enters a solid tube made of plastic having an index of refraction
of $1.60 .$ The light travels parallel to the upper part of the tube. (See Figure $23.58 . )$ You want to cut the face $A B$ so that all the light will reflect back into the tube after it first strikes that face.
(a) What is the largest that $\theta$ can be if the tube is in air? (b) If the tube is immersed in water of refractive index $1.33,$ what is the largest that $\theta$ can be?