# Physics307L:People/Dougherty/Photoelectric

SJK 00:57, 12 October 2007 (CDT)
00:57, 12 October 2007 (CDT)
overall, I think your data and analysis are good. I think a deficiency is in explaining the details of what you did, along with a better discussion of _how_ the systematic error could arise (not just where it may come from).

## Goal

SJK 00:53, 12 October 2007 (CDT)
00:53, 12 October 2007 (CDT)
Reading your "Goal" and "How it works," I do get the impression that you understood what was going on and what the lab was all about. However, a reader other than me (or someone who's done the lab) I think would have a difficult time understanding you. I think this could be improved by using more careful words (e.g., you say "work force" instead of "work function," which could confuse someone, since it's units of energy, not force), and also by taking more time to describe what is happening.

starting the first experiment, we want to determine in summary if it is more likely that light is in form of a wave or in photon form. the photon theory of light says that the energy of light is only determined by frequency and is independent of intensity. however, the wave thoery is only dependent on intensity. the first experiment is investigating this question.

the second experiment wants us to directly measure the constant h and the W0, work function, along with the total kinetic energy of the photon. with the total kinetic energy and work function of the photon we can find Planck's constant times the frequency. hv = KEm + W0

## How it Works

the photons emitted by the mercury light source enter the h/e apperatus and hit a cathode tube of electrons. the electrons escape the tube with a minimum energy or the work force W0. leaving the tube with a max kinetic energy. then by applying a reverse voltage on the energy, we can reverse the energy back to zero recording its stopping potential. this stopping potential is then directly related to the kinetic energy of the electron. plotting voltage vs. frequency we can get h which is planck's constant.

## Our Mean Value

SJK 00:55, 12 October 2007 (CDT)
00:55, 12 October 2007 (CDT)
I would say "uncertainty" or "uncertainty due to random error" instead of "mean error." Also, you should discuss here your systematic error! Since we "know" the "real" value, you should discuss the fairly large difference (compared with your uncertainty due to random error) from this value (as you mention in your lab notebook).

Overall, I think the whole summary needs more explanation: imagine that your reader hasn't ever seen the lab and doesn't know what you did, or what the "real" value of h is.
```    h = 7.13*10^-34     W0 = 2.48625*10^-19 J
```

with mean error

```    7.13*10^-34 +/- 1.02225242*10^-35
```

## reasons for errors

light filtering in from other sources

bad filters on the apparatus