Question

A flash light is covered with a filter that transmits red light. The electric field of the emerging beam is represented by a sinusoidal plane wave $\mathbf{E}_{\mathbf{x}}\mathbf{= 36}\mathbf{\sin}\mathbf{(}\mathbf{1.20}\mathbf{\times}\mathbf{1}\mathbf{0}^{\mathbf{7}}\mathbf{z}\mathbf{-}\mathbf{3.6}\mathbf{\times}\mathbf{1}\mathbf{0}^{\mathbf{15}}\mathbf{t)V/m}$. The average intensity of the beam will be

• A. $0.86\mspace{6mu} W/m^{2}$
• B. $1.72\mspace{6mu} W/m^{2}$
• C. $3.44\mspace{6mu} W/m^{2}$
• D. $6.88\mspace{6mu} W/m^{2}$

Answer 1

Answer: (B)

$1.72\mspace{6mu} W/m^{2}$

Answer 2

$I_{av} = \frac{c\varepsilon_{0}E_{0}^{2}}{2} = \frac{3 \times 10^{8} \times 8.85 \times 10^{- 15} \times 36^{2}}{2} = 1.72\mspace{6mu} W/m^{2}$