Lichtenberg figure, also known as an electron tree, beam tree, or lightning tree. Lichtenberg figures are branching electric discharges appearing on the surface or interior of an insulating material. This figure was created using an Electron Linear Accelerator (Linac), which delivered an electron beam aimed toward a thick block of acrylic. The electrons penetrate the surface of the acrylic, and rapidly decelerate as they collide with molecules inside the plastic, finally coming to rest. The pattern the electrons make in the acrylic as they rush out of the block looks like a tree, or fern-like structure. These figures can be found in the grass surrounding a lightning strike, and can also appear on the skin of lightning strike victims.