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11.Dual Nature of Radiation and matter
normal
A $160 \,W$ infrared source is radiating light of wavelength $50000 \,\mathring A$ uniformly in all directions. The photon flux at a distance of $1.8 \,m$ is the order of ............. $m^{-2} s ^{-1}$
A
$10$
B
$10^{10}$
C
$10^{15}$
D
$10^{20}$
(KVPY-2015)
Solution
(d)
Total photons emitted from source is
$N=\frac{\text { Power }}{h c / \lambda}$
$=\frac{160 \times 50000 \times 10^{-10}}{6.6 \times 10^{-34} \times 3 \times 10^8}$
So, photon flux falling over spherical surface of radius $r=18\,m$ is
$n=\frac{N}{A}=\frac{N}{4 \pi r^2}$
$=\frac{160 \times 50000 \times 10^{-6}}{6.6 \times 10^{-34} \times 3 \times 10^8 \times 4 \pi \times(18)^2}$
$\approx 10^{20} \,m ^{-2} s ^{-1}$
Standard 12
Physics
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