Question

how long would a laser have to be on to emit only one photon? Say the power of the laser is 1 W at 500 nm.

Answer 1

Given , the power of laser is 1 W or J/s.

i.e. in 1 second laser is emitting photons of energy 1 Joule.

The wavelength () of laser = 500 nm = 500 x 10^-9 m

We know that

  

Where, h = 6.63 x 10^-34 J-s is Planck's constant

E is energy of the photon

C = 3 x 10⁸ m/s is speed of the light

So, energy of a single photon emitting from the laser is  

According to the question, the laser is emitting 1 Joule of energy per second.

So, total number of photons (N) emitting from the laser in 1 second is :

I.e. time taken by laser to emit 2.5 x 10¹⁸ photons = 1 second

So, time taken by laser to emit one photon = 1 / (2.5 x 10¹⁸) = 3.978 x 10^-19 second

Hence, the laser have to be on for 3.978 x 10^-19 second to emit only one photon.

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