What determines the intensity of light?

I keep running into “intensity” whenever the subject of the Photoelectric Effect comes up.

Every author assumes I know what goes on, when one alters a dim light to bright. or to blinding. But I don’t.

Would someone please explain?

Ummm… more, or less photons are emitted (assuming for a moment that you believe in photons) - it’s not quite that simple, as the colour may change too and that also affects intensity.

But photons are present in all light from the dimmest to blinding glare.

The question is what do you do to light to bring it from dim to bright, etc.? What are the physics principles?

I think Mangetout meant by “more or less” was that when the concentration of photons in a given beam of light increases, so does the intensity. It’s not whether there are photons, but how many.

I do believe in photons. I do believe in photons. I do! I do!

You can count photons, and call that intensity. If so, it’s measured in Einsteins. 1 Einstein equals 1 mole of photons.
Or you can measure incident energy, say watts per steradian, and call that intensity.
Increasing either the number of photons (with a given energy distribution), or the number of watts per unit area amounts to an increase in intensity.

Okay. I have a halogen lamp illuminating my keyboard.

The intensity of the light, from dim to bright is varied by sliding the lamp’s switch from the rightmost position (dimmest) to the far left (brightest).

And all that happens, going from dim to bright is that the number of photons are increased?

Um, you emit more or less photons? I don’t seem to understand the question.

Say you have a filament as your photon source. You apply more energy, more atoms are excited, more photons are emitted, the intensity is higher. You apply less energy, less atoms are excited, less photons are emitted, the intensity is lower.

In physics terms (wrt the photoelectric effect) it’s just the number of photons incident on the surface in question, regardless of wavelength (although you might speak of the intensity within a certain range of frequencies).

The layman would view (literally) a light in yellow to blue area of the spectrum to be more intense than, say, one in the red area of the spectrum (for the same number of photons) but that’s just because the retina is more attuned (and thus, more sensitive) to those wavelengths than it is to longer wavelengths. (A strong ultraviolet source, while outside the visible range, could do permenant damage to the retina, but it wouldn’t be immedately apparent to the view, so I don’t know whether you’d consider that “intense” or not.)


Depending on the design, it is either alterning the AC characteristics or the resistance of the circuit, either way, less energy gets to the halogen filament, less atoms are excited, and less photons are emitted. This might change the temperature of the filament, altering the output light wavelength and also altering intensity through frequency, but that’s probably negligble

I know that the Wise Progenitors of the SDMB rejected the idea of subforums many, many moons ago, but couldn’t we have one just for mathematics and theoretical physics questions? Pretty please with sugar on top? :smiley:

I hasten to add that intensity is also dependant upon the area that the photons cover (or flux through). 500 watts distributed over an entire football field isn’t going to give you enough light to find your keys, but it’s quite enough to illuminate your living room.

So, intensity = number of phtons per unit area.


Nope, and that’s why the units used for intensity vary depending on what property of light you’re interested in.
As your light gets dimmer, its color temperature also changes. It stops putting so many blue photons, and emits more red photons. This shift is described in the wiki article on black body radiation.
To further complicate matters, your eyes are more sensitive to blue photons than they are to red, so your ability to read something placed under halogen lamp depends on the total number of photons and also the frequency distribution of the photons.

Complete hijak here, but Mangetout I’ve had proplems subscribing to your Blog.
Where, oh where did I go wrong?

I would really check out your site.

Maybe it’s easier to think in terms of a laser rather than any type of white-ish light. You know, since it doesn’t change color with power, and its intensity is always the same at any distance x from the source at power y.

So, yeah, it’s as simple as more power applied yields more photons output.

Brightness is something that’s not completely the same as intensity, although in sighted creatures such as ourselves it’s related. Consider an infrared laser at 5mW, a red laser at 4mW, and a green laser at 3mW. You’ll probably say that the green laser is the brightest of the three. Ok, I pulled the numbers out of my arse, but given the same power, green is brighter in aspect than red, and if you see the infrared, well, better hide yourself from the government – they’d probably be interested in you.

Thank you Balthisar — all the other contributors in this thread.

I now understand things a little better.

I’ve no idea. Actually I do have an idea… it may be that I’ve not turned on certain options/actions - I’m still getting to grips with the control panel… I’ll check it out tonight.