Q90 / 90 · KVPY 2015 (SB/SX) · Physics
A 160 watt infrared source is radiating light of wavelength $50000 Å$ uniformly in all directions. The photon flux at a distance of $1.8 \mathrm{~m}$ is of the order of
- A$10 \mathrm{~m}^{-2} \mathrm{~s}^{-1}$
- B$10^{10} \mathrm{~m}^{-2} \mathrm{~s}^{-1}$
- C$10^{15} \mathrm{~m}^{-2} \mathrm{~s}^{-1}$
- Correct answerD$10^{20} \mathrm{~m}^{-2} \mathrm{~s}^{-1}$
Solution
$N=\frac{I \lambda}{h C}=\frac{P \lambda}{\left(4 \pi R^{2}\right) h c}$
Hence, the correct answer is option: 'D'.