What happens in the photo effect when the light beam is very weak, but of sufficient frequency to emit electrons? I’ve heard of such an experiment, but I don’t have enough information about it.
To better explain the question I will quote a brief information from the book:
"The corpuscular nature of light is seen nicely when very low light is used, but enough frequencies for a photo effect to occur. Then the photo effect occurs after several minutes, just when one photon hits a small target (electron)."
I would like to know some more information about this experiment.
My internet search was unsuccessful.
I am particularly interested in whether after the photo effect occurs (after several minutes), the photo effect will take place continuously, or again it will take several minutes for the photo effect to occur.
I would expect it to take more minutes again for the next photo effect (depending on statistics). However, the experiment may not have confirmed this.
 A: A great explanation of the photoelectric effect and its significance can be found here.
I'm not sure what your level of understanding is, so I'll try to use some simple analogies and you can let me know if I'm aiming too high or too low.
In simple terms, Albert Einstein's famous experiment with the photoelectric effect consisted of exposing sodium metal to various frequencies and intensities of light and using a detector to measure the energy of the electrons emitted. "Frequency" describes how often a wave passes a certain point--think of how close together wavetops on the ocean are. That's frequency. Intensity is related to how much power a wave carries--think of the size of the waves on the ocean.
The graph below summarizes the results of Einstein's experiment:

The results of this experiment were quite surprising, and completely changed our understanding of the nature of light. To understand why, it's helpful to remember that at the time (1905), most physicists understood that light was a wave. In many ways, light does behave like a wave. Light has a frequency, and it carries energy just like a wave on the ocean; but this experiment showed that the wave theory of light couldn't be the whole story. Let's see why.
First, it was found that light below a certain frequency did not cause any electrons to be emitted, no matter how intense the beam of light was. Even if you cranked up the power and sent out a very high-intensity beam of light, no electrons would be 'knocked loose' if the frequency of the light was too low.
Second, the experiment showed that the intensity of the light increased the number of electrons emitted, but had no effect on their energy.
So, the key takeaways from this experiment were:

*

*A low-intensity beam of light can produce high-energy electrons if the frequency of the light his high.

*There is a minimum frequency below which even a high-intensity beam of light cannot produce a single electron.

*Increasing the intensity of the light increases the number
of electrons, but increasing the frequency of the light increases the energy of the electrons.

We would not expect these two things to happen if light was just a wave. Why not?
Waves carry energy as they travel. Light intensity is kind of like the "brightness" of a light bulb. A higher-intensity beam of light carries more energy--sort of like how a normal beach wave carries enough energy to topple a sandcastle, but a tsunami wave has enough energy to topple a city. In the same way, we might expect a high-intensity beam of light to result in higher-energy electrons. However, this is not what happens in the photoelectric experiment. If we applied the results of the photoelectric effect to our imaginary beach--that is, if we used 'toppling castles' as an analogy for emitting electrons, we would find that:

*

*A tiny, low-intensity wave could topple a city if the wavetops were close enough together.

*A giant tsunami could not topple a sand castle if the wavetops were too far apart.

*Increasing the intensity of the waves would increase the number of sandcastles we could knock over, but increasing the frequency is the only way to increase the size of building we could topple.

If this sounds crazy and bizarre, that's because it is. Waves on the beach don't behave this way. This is why the photoelectric effect was the first big clue that you can't totally explain the behavior of light by comparing it to a wave.
The best explanation for this behavior is that light sometimes acts like a wave, but sometimes it acts like little particles called 'photons'--sort of like extremely tiny ping-pong balls flying through space. Each photon carries a certain amount of energy that depends on its frequency. This is why low-frequency light can't knock electrons loose from sodium metal--the individual photons don't have enough energy! When you increase the intensity of light, you're just increasing the number of photons, not the energy of those photons. If none of these light particles has enough energy to eject an electron, it doesn't matter how many you add--it's just not going to happen.
This idea that light is both a wave and a particle is very important to understanding much of the world around us, and is crucial to physical sciences. It's one of those things where the evidence shows that light isn't really like anything we can compare it to in our every day life--it's kind of a unique thing that obeys its own rules.
Hope that helps!
