Crystals containing heavy atoms can diffract well, but also fluoresce themselves, causing interference. Gratings diffract each energy or wavelength present in the incoming radiation in a different direction. The wavelength to diffract is determined by the spatial frequency of the ribbons. As a result of the refractive index periodicity, these systems diffract light. As the beam propagates, it does not diffract, i.e., does not spread out. Relatively larger molecules like buckyballs were also shown to diffract. Lower frequencies diffract around large smooth obstacles such as hills more easily. In particular, massive particles can interfere and therefore diffract. Ocean waves diffract around jetties and other obstacles. Shoaling can also diffract waves, so the waves change direction. If the crystal is sufficiently ordered, it will diffract. Light hitting the zone plate will diffract around the opaque zones. However, the angle cannot be too sharp or the signal will not diffract. Lead increases the weight of the glass and causes the glass to diffract light.