Thin strips made of plexiglas and containing either two symmetric and collinear edge cracks or an edge crack close to a boundary were subjected to tension. If these strips were viewed in a coherent monochromatic light beam emitted from a He-Ne gas laser, the light reflected from the back surface of the strip deviated because of the highly strained zone surrounding the crack-tip and formed a caustic. It was previously shown [7] that the caustic is a circle, the diameter of which is directly related. to the stress intensity factor. The interaction between either symmetric cracks or a single crack and a boundary was studied by measuring the diameter and the distortion of the shape of the highly strained zones around the crack tips. A series of ratios of crack length to the minimum width of the strips were studied and the critical loads for initial of rapid fracture were determined and compared for either case.