Question

You'd like to double the number of photons in a beam of laser light with wavelength of 500 nm. How can you do this?

A.	Double the wavelenght of the beam, which you might do by doubling the number of times you pump the light-emitting material to reach excited states.

B.	Choose a light-emitting material with an excited state at twice the energy gap from the next lowest state, so that two photons are emitted in a single transition rather than one.

C.	Double the intensity of the beam, which you might do by doubling the amount of light-emitting substance in the laser (for instance, ruby).

D.	Double the frequency of the beam, which means photons are being emitted by twice as fast by the light-emitting material.

Answer 1

(A) If we double the wavelength then energy of each photon will become half as energy of a photon is inversely proportional to wavelength. But this will not change the no. Of photons.

(B) If we change the transition state then energy of each photon will change but this will change the no. Of photons emitted.

(C) If we double the intensity then no. Of photons will double , as intensity is propotional to number of photons.

(D) If we make the frequency double then energy of each photon will doubled as energy is proportional to frequency.

So correct answer is (C)