# Understanding Intensity and Energy Relationships

• zmike
In summary, doubling the frequency of photons increases their energy by a factor of four. This increase in intensity is due to the fact that each photon has twice as much energy.f

#### zmike

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

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??

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...

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.

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.