# I Photon Interval vs Areal Density for Changing Intensity

1. Aug 9, 2016

### greswd

When one reduces the intensity of let's say, an incandescent bulb (by varying the resistance, as seen in many homes), which decreases more, the photon frequency (not related to wavelength, but the time interval between photon emissions) or the areal density of the photons?

To what extent does each decrease?

2. Aug 11, 2016

### ceil

The emissions of photon related to the frequency.

3. Aug 11, 2016

### ceil

The intensity just can increase or decrease the number of photons, but the movement enargy of photons related to the frequency of light

4. Aug 11, 2016

### greswd

I don't understand what you're saying.

5. Sep 2, 2016

6. Sep 2, 2016

### Jilang

If the frequency of emission decreases the aerial density will do too. By the same proportion.

7. Sep 2, 2016

### greswd

thanks. do you have any literature on this?

8. Sep 2, 2016

### Jilang

No. It's just a consequence of flux. If you double the flux you double the rate of that the photons pass through a shell. Given that they all travel at c, the speed of light, the density in the shell must be double.

9. Sep 2, 2016

### greswd

No, you can double the flux by halving the interval and leaving the density unchanged. The photons will still travel at c.

I want to know the density and interval relationship for an incandescent bulb, or any other type of light source you know of.

10. Sep 11, 2016

### Nosebgr

Incandescent light bulbs emit light as a result of heat. We can treat the bulb as a black body source. The question you ask is really specific, I will answer according to my intuition however if I am mistaken feel free to correct.

A black body source emits light as a result of collisions between atoms and molecules in the atomic structure. As the material heats up, the atoms and molecules start to collide with higher kinetic energies, thus in a given time frame more collisions occur. We can roughly treat each collision as an emission of a single photon. Therefore the rate of collisions is the rate of photon emission.

By areal density, I am assuming you mean the number of photons per unit area perpendicular to the radial emission (photon's travel path). We can then say that the areal density of of photons is roughly the number of collisions happening per unit area. Then no matter the temperature of the material, the areal density will remain constant - since the number of atoms, thus the number of collisions is constant.

So, at low temperatures the emission rate decreases, but the number of photons per emission (if it makes sense) remains constant.

11. Sep 17, 2016

### greswd

@Jilang, according to Nosebgr, depending on the system's dynamics, the areal density can be constant.

12. Sep 18, 2016

### Jilang

Planck' law says that the higher the temperature of the body the more radiation it emits at every wavelength. There is one photon for each emission. The rate of emission therefore cannot be constant.

13. Sep 18, 2016

### greswd

Nosebgr didn't say that it was constant.

14. Sep 18, 2016

### Jilang

I read his penultimate paragraph to say that the number of collisions was constant and the number of emissions was constant. The final paragraph seemed to contradict this though. Perhaps he can clarify this.