$33\mathrm{.}17-$ Light Pipe. Light

Figure E$33\mathrm{.}17$

enters a solid pipe made of plastic having an index of refraction of $1\mathrm{.}60.$ The light travels parallel to the upper part of the pipe $($Fig$.$ E$33\mathrm{.}17).$ You want to cut the face $AB$ so that all the light will reflect back into the pipe after it first strikes that face. $($a$)$ What is the largest that $$ can be if the pipe is in air? $($b$)$ If the pipe is immersed in water of refractive index $1\mathrm{.}33,$ what is the largest that $$ can be$?$