# E=hf Confusion

This is a sentence in my textbook:
"
E=hf

Do not be misled by this equation: This equation gives the energy per photon. It turns out that if we double the fequency, we also double the number of photons increasing the intensity by a factor of four as expected.
"

I do not understand this last sentence. Is there some sort of equation relating intensity to energy because I thought intensity was just the number of photons :S

Anyone want to help?

thanks

## Answers and Replies

intensity is also power per unit area... in this case a more energetic photon, is a more intense photon.

is there a formula? how did they get 4x more intense??

Pengwuino
Gold Member
The book isn't making too much sense. The intensity is Power/Area. The energy of a photon is E=hf. The power would be hf/tA With t being the time interval and A being the area we're looking for the intensity at. What subject is the book talking about? As far as I can tell, some variable doubling increases simultaneously the intensity and energy of the individual photons. Maybe you're talking about blackbody radiation and Wien's displacement? Although with the T^4 temperature dependence of what I'm thinking could be what you're talking about, that sentence still wouldn't make sense...

Doc Al
Mentor
This is a sentence in my textbook:
"
E=hf

Do not be misled by this equation: This equation gives the energy per photon. It turns out that if we double the fequency, we also double the number of photons increasing the intensity by a factor of four as expected.
"
As a stand alone statement, it doesn't make sense. But what is the context? What textbook is it from and what is being discussed?

As a stand alone statement, it doesn't make sense. But what is the context? What textbook is it from and what is being discussed?

This is from a MCAT prep book, in the Chem>atomic structure>photoelectric effect section.

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My take on it is this:

If you double frequency, you DOUBLE the number of photons passing a given point per second.

According to E=hf, you ALSO DOUBLE the energy of each individual photon.

2x energy per photon, and 2x amount of photons gives 4x total energy.

Doc Al
Mentor
My take on it is this:

If you double frequency, you DOUBLE the number of photons passing a given point per second.
Doubling the frequency doesn't change the number of photons.

My bad.

As written above the statement isn't sensible, however, perhaps someone made a typo and meant to write the following, which would make sense: If you do two separate things, if you double the number of photons emitted per second, and if you also double the frequency so that each photon will have twice as much energy, then you will have four times the intensity.

Doc Al
Mentor
however, perhaps someone made a typo and meant to write the following...
I'm thinking the same thing. Just changing a few words would turn the sentence into something sensible. That's why I wanted the exact reference.