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Question:

A 2 mW laser operates at a wavelength of 500 nm. The number of photons that will be emitted per second is :[Given Planck's constant h = 6.6 × 10^-34 Js,speed of light c = 3.0 × 10^8 m/s]

1×1016

1.5×1016

5×1015

2×1016

Solution:

Correct option is B. 1.5×1016
As we know that Energy is given by
E=hc/λ
Here P is the power
P=n.hc/λ
n=Pλ/hc =2×10^-3×500×10^-9/6.6×10^-34×3×10^8 =1.5×10^16