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Shadows might seem like a trivial optical phenomenon, but they actually contain a variety of brightness and color if you observe carefully. This chapter focuses on the primary visual features that make watching shadows both entertaining and informative. To begin, a shadow is a region of space devoid of direct illumination. However, the sun is not a point source but has an angular width near 0.5°, so it casts a dark shadow (the umbra) surrounded by a partially illuminated shadow (the penumbra). An exciting example is a solar eclipse (Fig. 6.1), in which the shadow of the moon blocks direct sunlight from reaching the earth.
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