Equipment:
Theory: When light at non-normal incidence crosses an interface between two media of different refractive index, it will be refracted. In this case, the light travels from air to a medium of higher refractive index, so it will be refracted towards the normal.
Description: Mount the rectangular block and the 5-ray box on the blackboard, powering the ray box with as much as 12 V DC. It might be easier to use three rays rather than 5, so the outer two can be eliminated by changing the angle of the deflecting mirrors. The angle of incidence can be adjusted by changing the orientaion of the block. The refraction of the light at the two interfaces should be clearly seen; point out that the light emerging from the block is parallel to the light entering the block.
Setup time: 5 minutes.