Rotate the semicircular slab of plexiglass with the light ray entering and exiting through the curved surface. Starting with the ray incident nearly perpendicular to the flat internal surface, rotate the semicircular slab until the critical point is reached and total internal reflection is obtained. Reflection at entrance can be avoided by having the light enter perpendicular to the surface. A single slit baffle and convex lens are used to produce a narrow ray of light.
The Brewster angle can also be demonstrated by rotating the semicircular slab so that the transmitted refracted ray and the internally reflected ray are perpendicular. See Demonstration M7-11: OPTICAL BOARD - BREWSTER'S ANGLE.