The spectral emissive power $E_\lambda$ for a diffusely emitting surface is $E_\lambda = 0$ for $\lambda > 3 \mu m$; $E_\lambda = 150 W/m²\mu m$ for $3 < \lambda < 12 \mu m$; $E_\lambda = 300 W/m²\mu m$ for $12 < \lambda < 25 \mu m$; $E_\lambda = 0$ for $\lambda > 25 \mu m$. The total emissive power of the surface over the entire spectrum is
- A. 1250 W/m²
- B. 2500 W/m²
- C. 4000 W/m²
- D. 5250 W/m²
Answer: D. 5250 W/m²
Explanation: Total power = $\int E_\lambda d\lambda = 150(12-3) + 300(25-12) = 150(9) + 300(13) = 1350 + 3900 = 5250$.