# Q & A: Photon number, energy, and wavelength

Q:
Why the number of the photons decreases when its energy increases ?
Egypt
A:

Your premise about the number of photons and energy is not quite correct.  The relation that we now believe to be correct is that the wavelength of the photon decreases as the energy of the photon increases.   They are related by  λ = hc/ Ephoton where h is Plank's constant, c is the velocity of light and λ is the wavelength of the photon.
In a given packet of photons the total energy is proportional to the number of photons.

LeeH

(published on 05/06/2012)

## Follow-Up #1: More on energy and photon number.

Q:
Firstly, Thanks for answering me so fast. The total energy of photons is what I meant. I read in a book the following paragraph and there was no explanation: "The emission is a great flood of photons, their energy increases when their frequency increases. But their number decreases when the energy increases" Is it incorrect ?
Egypt
A:

The first statement is correct.   The only way the second statement is correct is if the writer implied that "For a constant amount of total energy,  the number of photons would decrease as the frequency increases".  Otherwise it's wrong as explained in the previous post.

LeeH

(published on 05/07/2012)

## Follow-Up #2: why is photon energy hf?

Q:
why number of photon is greater in red light than in blue light,assuming their energies equal?please tell me theoritical reason .thanks
- hajra naeem (age 18)