# Homework Help: Light Intensity and Aperture Variable for Planck's Constant

1. Aug 13, 2009

### Procrastinate

I used a Planck's constant kit and varied the frequency of light in order to verify Planck's constant.

However I have also used apertures to vary the surface area and I am quite confused as to what to do with this data to relate it back to Planck's constant.

The data is as seen in in the attachment. I'm sorry they are neat and tidy, they are currently just rough drafts at the moment. I had to leave out the blue light because it wouldn't let me go over 300kb.

Is there anyway i can use light intensity as a variable in order to verify Planck's constant? Or any other thing similar to this.

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• ###### Physics Graphs Planck.doc
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2. Aug 13, 2009

### Delphi51

I am not familiar with your "Plank's constant kit"; it would be useful to know what it is. Anyway, you must be shining light on a photoelectric tube and measuring the electric potential needed to stop the current from flowing - your "backing voltage"? The next step is to compare your data with Einstein's formula for the photoelectric effect (graph it so the formula for the line of best fit matches Einstein's formula). Use the form of the formula where the kinetic energy of the electron is replaced by qV, the electric energy that is absorbed in stopping it. You will notice that light intensity is not in the formula, so you cannot find Planck's constant using intensity. But you can find it by fitting your data to the formula. Look up "photoelectric effect" in Wikipedia if you don't have it handy in your textbook.

3. Aug 13, 2009

### Procrastinate

http://www.scientrific.com.au/PDFs/ap2341-001.pdf

This is basically the instruction manual for what it does. Einstein's formula

I looked it up on Wikipedia and found that Einstein's formula was

However, I'm not sure how to apply it with the data I have currently.

Last edited by a moderator: Apr 24, 2017
4. Aug 14, 2009

### Delphi51

The kinetic energy of the electrons emitted from the photo tube's cathode can be determined by putting an electric potential across the tube. The Ek is equal to the electrical energy absorbed, qV when V is adjusted to just the voltage necessary to stop the electrons so no current flows. So you have
hf = W + qV where W is the work function of the cathode (energy needed to push one electron out)

The trick is to rearrange this formula so it represents a graph where it is convenient to find the h value you are looking for. It would be nice if h was the slope on the graph. Recall that in the math formula for a straight line, y = mx + b, m is the slope. Also nice if the manipulated variable (the thing you set in the experiment) is on the right. And the responding variable - stopping voltage - is on the left. Once you have this sorted out, you will know what to graph. If your formula was y = mx + b, you would put x on the horizontal axis and y on the vertical axis. According to Einstein's theory, you should get a straight line to within experimental error and if all goes well the slope will be Planck's constant to within experimental error. It would be nice if you had an error estimate on your measured values so you get error bars on the graph. Then you can tell if it is a straight line to within experimental error.