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As you will demonstrate in your lab this week, the following two graphs can be obtained from the photoelectric effect experiment. In these graphs, "I" represents the electric current.

In the current vs frequency graph, we see that the electric current only start once some threshold value of frequency is used.

Current vs voltage graph

The figure "Current vs Voltage" shows another strange aspect of the photoelectric effect. As you increase the intensity of the light, you get a bit more current but the stopping potential (and thus the kinetic energy of each electrons) does not seem to change.

Fig. "Current vs Frequency" shows that there is a threshold frequency f_0 below which nothing happens. Fig. "Current vs Voltage" shows that, for positive potential, I can have a large current, and for any intensity of the light, the stopping potential V_{stop} remains unchanged.

The standard idea of physics that light is a wave that excites the electrons " to jump out of the swimming pool" explains nicely why the current (denoted I in the figures) is proportional to the intensity, but some of these experimental facts beg the following questions (which we will answer).

1. Why is there a threshold frequency? Why does it matter what color the light is?
2. If the light is more intense, should it not give more energy to the electrons? Should we not then need a higher potential V_{stop} to stop them? Why is V_{stop} the same for weak or strong light intensity?

The answers to these questions, once figured out, changed the way we look at the world. In your lab this week, you will be asked to reproduce these graphs and to perform the photoelectric experiment. The demos below will demonstrate what the actual experiment looks like.

Intensity

First let's look at what happens when we change the intensity.

Explanation Follow-up

Frequency

Now, the strangest part is the frequency dependence.