# Intensity, the Photoelectric Effect & Light-as-a-Particle **URL:** \ **Category:** Factual Questions\ **Created:** [October 15, 2007, 6:28pm UTC](https://boards.straightdope.com/t/intensity-the-photoelectric-effect-light-as-a-particle/422818 "2007-10-15T18:28:22Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Omoikane](https://avatars.discourse-cdn.com/v4/letter/o/c4cdca/32.png) [@Omoikane](https://boards.straightdope.com/u/Omoikane)\ **Post date:** [October 15, 2007, 6:28pm UTC](https://boards.straightdope.com/t/intensity-the-photoelectric-effect-light-as-a-particle/422818/1 "2007-10-15T18:28:22Z")
Thinking out loud here, I’ve been very confused for at least the past hour. I think my ultimate question is what’s in bold at the bottom. Very simple ( 😉 ) request - can someone explain how the photoelectric effect leads away from seeing light as a wave and towards seeing light as a particle? I’ve been going through all the Google and Wikipedia entries on the subject, and none of them are really clear enough. Here is a quote about the experiment of one Phil Lenard: > [@](#): > > He used a bright carbon arc light to study how the energy of the emitted photoelectrons varied with the intensity of the light and, by separating out individual colors, with the frequency of light. Increasing the frequency of light, by selecting light from the bluer end of the spectrum, caused the ejected electrons on average to be more energetic, as predicted – because it was assumed they’d been made to vibrate faster. Increasing the intensity of light (by moving the carbon arc closer to the metal surface) caused more electrons to be thrown out, also as expected. On the other hand, increasing the intensity had no effect at all on the average amount of energy that each ejected electron carried away. That came as a real shock. If, as physicists believed, the photoelectric effect followed from an interaction between electrons and electromagnetic waves, then intensifying the radiation ought to shake the electrons in the metal surface harder and so shoot them out with more energy. It was a mystery why this didn’t happen. Maybe my confusion here is about the relationship between energy, intensity and frequency in light. **Now increasing the intensity of the light means increasing the amount of energy that it transmits, yes? So why doesn’t that increase the energy of the emitted electrons? Or does intensity just mean you change the amount of light being emitted? In which case, why would the amount of light being emitted cause the electrons to be emitted more energetically? Wouldn’t you just expect there to be more of them, as per the results of the experiment?** ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [October 15, 2007, 6:58pm UTC](https://boards.straightdope.com/t/intensity-the-photoelectric-effect-light-as-a-particle/422818/2 "2007-10-15T18:58:51Z")
The part you’re missing is that the systems are quantized – there is a certain energy required to kick and electron out. Below that energy, no electronss get kicked up to the conduction band, regardless of how intense the light is. (multiphoton effects are negligible until you get to monumental intensities, like what you get from a high-power laser). The photoelectric effect is one of many effects that suggest that energy is quantized. If the light energy is carried in the form of a particle, it’s easy to see how it’s quantized – so much energy per photon. Increasing intensity means increasing the number of these photons. And if one photon per electron is needed to induce the photoelectric effect, it all fits together, because if the energy per photon is insufficient, then changing the number of them you throw at the metal still won’t increase the number of libearted electrons. If light is a wave, on the other hand, then , regardless of the energy, I can eventually get the energy delivered by the wave up to that needed to liberate an electron by simply dialing the intensity up. ---
**Author:** ![RickG](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rickg/32/5174_2.png) [@RickG](https://boards.straightdope.com/u/RickG)\ **Post date:** [October 15, 2007, 7:36pm UTC](https://boards.straightdope.com/t/intensity-the-photoelectric-effect-light-as-a-particle/422818/3 "2007-10-15T19:36:18Z")
A further ‘amplification’, if you will. If light is strictly a wave, then increasing the intensity increases the amplitude of the wave. The amplitude is the maximum strength of the electromagnetic field of the wave. So, you’d expect a stronger field to accelerate the electrons more, thus causing them to be ejected with higher energy. Also, you’d expect that you could still eject electrons with “red” (lower energy) light, if the intensity were high enough. But, as it turns out, while the field strength does increase with increasing intensity, it does that by having more photons. Thus, more emitted electrons, but all at the same energy as before. And low energy (low frequency) light can’t eject an electron, no matter how intense, because the energy is absorbed one photon at a time. Does that make sense? -Rick ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 15, 2007, 8:24pm UTC](https://boards.straightdope.com/t/intensity-the-photoelectric-effect-light-as-a-particle/422818/4 "2007-10-15T20:24:27Z")
Or another way to put it: If I have a single photon, with almost enough energy to liberate an electron, then it’ll almost liberate an electron. If I instead have two photons with that energy, they still won’t liberate an electron; instead, they’ll almost liberate two different electrons. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [October 15, 2007, 9:00pm UTC](https://boards.straightdope.com/t/intensity-the-photoelectric-effect-light-as-a-particle/422818/5 "2007-10-15T21:00:31Z")
A very nice layman’s explanation can be found here. [http://www.colorado.edu/physics/2000/quantumzone/photoelectric.html](http://www.colorado.edu/physics/2000/quantumzone/photoelectric.html) ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [October 16, 2007, 10:42am UTC](https://boards.straightdope.com/t/intensity-the-photoelectric-effect-light-as-a-particle/422818/6 "2007-10-16T10:42:06Z")
My understanding was that the problem with the photoelectric effect and wave light is that the photoelectricity starts to flow as soon as the light hits the target, and does not wait until enough uniformly distributed energy would have hit the region occupied by an electron. Therefore the energy must arrive in packages.