27. Let f be the fraction of the incident beam intensity that is reﬂected. The fraction absorbed is 1 − f . The reﬂected portion exerts a radiation pressure of p r = 2 fI0 c and the absorbed portion exerts a radiation pressure of p a = (1 − f ) I0 c , where I0 is the incident intensity. The factor 2 enters the Frst expression because the momentum of the reﬂected portion is reversed. The total radiation pressure is the sum of the two contributions: p total = p r + p a = 2 fI0 +(1 − f ) I0 c = (1 + f ) I0 c . To relate the intensity and energy density, we consider a tube with length ` and cross-sectional area A , lying with its axis along the propagation direction of an electromagnetic wave. The electromagnetic energy inside is
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