# Photoelectric Effect (Applications)

Introduction

At the end of 1800 years, more precisely around 1887 a renowned German physicist (we know today his name from the measuring unit of frequency) H.R. Hertz, performed for the first time an experiment that will change the world today as we know it. This experiment changed not only our physics understanding of some fundamental aspects of the nature, but also constituted the starting point in developing many devices that we know now: the solar cells, the video cameras, the photo-detectors, etc. What Hertz discovered in his experiment from 1887 was named the PHOTOELECTRIC EFFECT.

Physics of the photoelectric effect

In the original experiment, an ultraviolet light was shined on two metallic electrodes that had an electric potential between them. Hertz observed that when the ultraviolet light was on, the potential necessary to produce the spark was diminished. Because in this experiment were involved light and also electricity he named this effect PHOTOELECTRIC. The explanation that Hertz gave to this effect is the following: when light shines on the metal, electrons are emitted from the atoms present on its surface. They are named photo electrons and they give a photo current which intensity can be detected and measured. Further studies of the photoelectric effect by Phillip Lenard revealed the four laws that apply to this phenomenon.

1.   The intensity of the photo current is proportional to the intensity of the light, for a certain material. That is, a brighter light shined on the surface gives a bigger intensity of the photo current.

2.   The maximum kinetic energy of the electrons emitted is proportional to the frequency of the light and does not depend on its intensity for the same material. That is, the intensity of the photo current is bigger for the green color than for the red color of light.

3.   There is a threshold in light frequency for producing the photoelectric effect. (This threshold depends on the material and can be measured by applying a reverse voltage on the electrode U_0). That is, for every substance there is no photoelectric current detected under a certain frequency of light (green light can emit electrons while red light cannot, depending on material).

4.  The photoelectric effect is instantaneous. That is, the photoelectric current is produced at the same moment when light is applied.

5.  The intensity of the photo current does not depend on temperature that is, this effect is different from the thermionic emission.

The explanations for these facts were given in 1905 by Albert Einstein. For his corpuscular theory of light, he was awarded in 1921 the Nobel Prize. The main assumption of the corpuscular theory is that light has a particle character, in this effect.  These particles of light were named photons. The energy of a photon is proportional to its frequency (and inversely proportional to its wavelength):

$E_{photon}=hν=h c/λ$      ($ν$ is the frequency,$λ$ is the wavelength)
Thus the first two laws of the photoelectric current can be written in mathematical symbols as

$hν=E_k+ W$  where

$\left\{\begin {matrix} hν \text{ is the energy of the incident photon}\\E_k=(m v^2)/2\text{ is the kinetic energy of the electron}\\W=e U_0 \text{ is the work necessary for emission of one electron (work function of material)}\end{matrix}\right.$

together they give

$hν=(m v^2)/2+e U_0$

Because the effect was produced at the surface of the material (the photo electrons were emitted from the atoms of the surface), it was named EXTERNAL PHOTOELECTRIC EFFECT. Somehow latter, when studies of semiconductor devices began, it was noticed that this effect can take place also in the interior of some substances (semiconductors). Thus the release of an electron-hole pair by a photon that is absorbed in the bulk of a semiconductor is named INTERNAL PHOTOELECTRIC EFFECT. This effect is responsible for how the solar cells work.

Positive aspects

There are many considerations that we have to take into account when talking about solar cells. Some of them are positive, some of them are negative, but the big number of positive aspects recommends the solar cells for the future of the energy generation. First and more important is the fact that solar energy is everywhere. Because of this, solar energy is sustainable, or in other words it can satisfy all necessities for the present and future. Solar energy is friendly with the environment and because of this the pollution is reduced.  Solar panels reduce the cost of electricity and increase the home safety. They make electricity available everywhere, even in the most remote locations. It stabilizes the economy allowing us to reduce the consumption of oil. The production of solar energy is clean and silent. After they are installed solar panels do not require extensive maintenance. Governments and states support the development of solar energy.

Negative aspects

There are a small number of negative aspects that come with all the solar energy benefices.  Solar panels are expensive to produce and they require large spaces and exotic materials. These materials are expensive and rare in nature. In fact the cost of solar panels is the main factor for which until now solar energy generation is behind other methods of producing electricity. Then, because sun does not shine all day, the solar energy is intermittent. Therefore it is necessary a backup over night when light is not available. Energy storage facilities are very expensive and this is the second fact because solar energy generation is not so widely spread. The third fact that is against solar energy is the fact that the manufacturing and transport of solar panels produces pollution. In some countries, because of the weather and unreliable climate, the produced solar energy is negligible. Solar stations require large spaces of land that can be used for other purposes.

Conclusion

In conclusion, a physics experiment done by Hertz at the end of the 1800 years allowed the discovery of a very important physics effect that we call today, the PHOTOELECTRIC EFFECT. The explanation of the photoelectric effect changed the physics. Today we use the photoelectric effect in many devices such as solar cells. The solar energy generation is one of the most promising methods for producing the energy of the future and has many positive aspects. There are however also some facts that are negative, and because of these the production of solar electricity is not very widespread as it should be.

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