An example of this would be of light traveling through water towards the boundary with a less dense material such as air. When the angle of incidence in water reaches a certain critical value, the refracted ray lies along the boundary, having an angle of refraction of 90-degrees. This angle of incidence is known as the critical angle; it is the largest angle of incidence for which refraction can still occur. For any angle of incidence greater than the critical angle, light will undergo total internal reflection. So in laymen's terms, the critical angle is defined as the angle of incidence which provides an angle of refraction of 90-degrees.
The umbra is the darkest part of a shadow. From within the umbra, the source of light is completely concealed by the occulting body. In astronomy, an observer in the umbra is said to be experiencing a total eclipse.
The penumbra is the region in which only a portion of the occulting body is obscuring the light source. An observer in the penumbra experiences a partial eclipse.
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