Equipment:
Theory: When light travelling in one medium hits the boundary of a medium of lower refractive index, it will be completely reflected as long as the angle of incidence is larger than the critical angle. The sine of the critical angle is equal to the ratio of the refractive indices, n2/n1, with n2 less than n1.
Description: Mount the rectangular block and the 5-ray box on the blackboard, powering the ray box with as much as 12 V DC. In this case, it might make the situation less complicated if only the central ray is used. The other four can be removed by changing the orientation of their aligning mirrors. At small angle of incidence, light will be transmitted from the block to air. At higher angles of incidence no light will be transmitted.
Setup time: 5 minutes.