Light and seeing
Light passes through transparent material (for example, glass) but not through opaque material (for example, wood). If an opaque object is placed in front of a point light source, in a darkened room, a sharp edged shadow is formed in the region of less intense light — indicating that light travels in straight lines (see Figure 17.1). You can imagine light spreading from the point source in all directions, as rays, in straight lines. However, in a diagram (such as Figure 17.1) only those rays needed to show the position of the shadow are represented.
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