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# Stuck here, help me understand: monochromatic light of wavelength 667 nm is produced by a helium neonlaser. The power emitted is 9 mW. The number of photons arriving per sec on the average at a target irradiated bt this beam is:

monochromatic light of wavelength 667 nm is produced by a helium neon laser. The power emitted is 9 mW. The number of photons arriving per sec on the average at a target irradiated bt this beam is:

• Option 1)

$3\times 10^{16}$

• Option 2)

$9\times 10^{15}$

• Option 3)

$3\times 10^{19}$

• Option 4)

$9\times 10^{17}$

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As we discussed in concept

photon -

A packet or bundle of energy is called a photon

-

Power of EM wave is $P=\frac{Nhc}{\lambda}$

N = number of photon arriving per second

= $\frac{P \lambda}{hc}$

N = $\frac{9\times 10^{-3}\times 667\times 10^{-9}}{6.62\times 10^{-34}\times 3\times 10^{8}} = 3 \times 10^{16}$

Option 1)

$3\times 10^{16}$

This option is correct.

Option 2)

$9\times 10^{15}$

This option is incorrect.

Option 3)

$3\times 10^{19}$

This option is incorrect.

Option 4)

$9\times 10^{17}$

This option is incorrect.

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