Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Photoelectric Effect and Photons as Particles
Name: Timothy
Status: student
Age: N/A
Location: N/A
Country: Great Britain
Date: N/A


Question:
Why does the photoelectric effect prove photon particles?

I was reading about the photoelectric effect of light, and lights duality. It appeared that most everything can be explained by the wave aspect except for this effect. Anyway something came to mind I do not understand. Why would the photoelectric effect prove the particle aspect of light? I know that the frequency is key to effect and not the amplitude, but this seems more like a wave effect then a particle effect. I know when I ride my boat I like the big rolling waves. My boat rides over them with easy because the up and down movement is not so sudden, the height of the wave does not seem to matter as long the wave is long enough. In fact I sometimes do not notice them at all. However when the water is choppy meaning there is a whole bunch of small amplitude waves at high frequency hitting the boat, I come home with a sore butt, and everything in the boat get tossed around all kinds of ways, do to constant up and down movement. So it would seem that amplitude is not always as destructive as the frequency.

So if it is waves that knock me around why does it have to be photons that knock atoms around, why not just the energy in light waves?



Replies:
First, I like how you think. Thinking about yourself in the place of a particle (or any other subject of study) is a great way to analyze scientific subjects.

Unfortunately, quantum effects tend to defy traditional notions of 'how things work'. In this example, light waves differ from water waves in many important ways. In the photoelectric effect, light hits some material which absorbs it, and then ejects incident electrons. The reason that the photoelectric effect is evidence for the particle nature of light has to do with how materials absorb that light energy and then eject it in the form of electrons.

As you have read, scientists believe that light consists of individual packets called photons, with discrete amounts of energy (they are also called 'quantized', or made of quanta). For now, I'd like you to forget that. For the sake of argument, suppose light is purely waves, not photons. If light were not quantized, you could create light of any energy intensity you wanted regardless of the frequency. For instance, you could take some light of one frequency and adjust its intensity such that it equals the intensity of light of a different frequency. If this were true, using the photoelectric effect (where light is converted to electrical current), you could change the energy of incident electrons purely by changing the intensity of light, regardless of its wavelength.

It turns out, experiments do not support this. You cannot affect the energy of incident electrons by changing the intensity of light. You can eject more of them, but they turn out to all have the same energy. In contrast, the frequency is very important in the photoelectric effect. The only way to increase the energy of the electrons ejected in the photoelectric effect is to adjust the light's frequency. This shows that there is something about the nature of the light, not just how much of it there is, and suggests that light cannot be infinitely divisible as pure waves would be.

So it turns out that photovoltaics, like much of quantum physics, are not very much like your boat or you butt. :)

Hope this helps,

Burr Zimmerman



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory