KVPY SX SB Exam 2015 Question Paper

Question : 37 of 120

Marks: +1, -0

A 160 watt infrared source is radiating light of wavelength 50000Å uniformly in all directions. The photon flux at a distance of 1.18 m is of the order of -

$10 \m^{-2} \s^{ -1}$
$10^10 \m^{-2} \s^{ -1}$
$10^15 \m^{-2} \s^{ -1}$
$10^20 \m^{-2} \s^{ -1}$

Validate

Solution:

P = 160 watt, λ = 50000Å

I=P/{4πr^2}

nh υ =P/{4πr^2 }

⇒

n=P/{4πr^2(h υ)

(n = no. of photons per sec per m

^2

)

n ={Pλ}/{4πr^2 hc

n = 10^20 m^{-2} s ^{ -1}

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