Source : Département de physique de Yong P. Chen, PhD, & & astronomie, Faculté des sciences, Université de Purdue, West Lafayette, Indiana
Effet photoélectrique désigne l’émission d’électrons par une metalwhen lumière brille sur elle. Afin que les électrons d’être libérés du métal, la fréquence de la lumière doit être suffisamment élevée tels que les photons dans la lumière ont une énergie suffisante. Cette énergie est proportionnelle à la fréquence de la lumière. L’effet photoélectrique a fourni la preuve expérimentale pour le quantum de lumière qui est appelée photon.
Cette expérience démontrera l’effet photoélectrique en utilisant un métal zinc chargée sous réserve soit une lampe ordinaire, ou la lumière ultraviolette (UV) avec une fréquence plus élevée et l’énergie des photons. La plaque de zinc sera reliée à un électroscope, un instrument qui peut lire la présence et la quantité relative de charges. L’expérience démontrera que la lumière UV, mais pas la lampe ordinaire, peut s’acquitter le zinc chargé négativement par éjecter les électrons en excès. Aucune source de lumière, cependant, peut s’acquitter de zinc chargée positivement, compatible avec le fait que les électrons qui sont émises dans l’effet photoélectrique.
For steps 2.1-2.4, the electroscope remains charged (needle remain deflected) for both the regular lamp and UV light illumination (Figure 2b and 2c), indicating that the zinc plate remains positively charged.This is because the charged zinc plate (which has already lost some electrons in the first place to become positively charged) further losessome photoelectrons by the UV light to make it further positively charged. In thiscase, itmay be noticeablethat the needle of the electroscope deflects a bit further in Figure 2c.The regular visible light doesnot change the positive charges on the zinc plate and the electroscope remains charged as well (Figure 2b).
For steps 3.1-3.5, when the zinc plate is negatively charged, it can be observed that the regular lamp light again has no effect on the electroscope (Figure 3b), while the UV light causes the needle of the electroscope to collapse and return to the uncharged position with no deflection, Figure 3c. This is because only the UV light photons have enough energy (above the workfunction of zinc) to eject photoelectrons, thus to discharge the zinc that has been previously charged to be negative (with excess electrons).
In this experiment, we haveused an electroscope to show that UV light can discharge a negatively charged zinc metal through the photoelectric effect.In contrast, a positively charged zinc sample (which has already lost some electrons) will not be discharged, nor will a visible light (which cannot cause the photoelectric effect) discharge either negatively or positively charged zinc.
The photoelectric effect played important roles in the development of quantum physics in the 20th century as it provided experimental evidence that light is made of particles that we call photons andcarry quanta of the light energy proportional to light frequency.
Practically, the photoelectric effect has also been used to make various optoelectronic devices, such as photosensitive electrical switches-where the blocking or unblocking of a light beam shining on a metal turns off or on an electrical current due to the absence or presence of photoelectrons.This is commonly used in many mechanical-position sensors (for example opening or closing of a door that unblocks or blocks a light beam).
The photoelectric effect is a basic physical phenomenon that not only has a variety of practical current-day applications, but has also inspired a whole new field of science.
A metal contains many mobile electrons. These electrons can be excited when provided with energy. And, if the energy is high enough, the electrons can be excited out of the metal.
When such an excitation is made with light, the ejected electrons are known as photoelectrons, giving this effect its name – the photoelectric effect.
Here, we will demonstrate the photoelectric effect using a charged zinc metal plate that is subjected to regular lamp light and ultraviolet light.
Before we learn how to perform the experiment and collect data, let’s discuss the parameters and principles that govern this effect. It has been observed that in order for the photoelectric effect to happen, the frequency ‘f’ of the light has to exceed some minimal threshold ‘f0’ (read- f-zero).
To understand why this is important, let’s zoom in and take a look at this process at the microscopic level. When the light is shone on a metal, individual light photons are absorbed by the electrons in the metal. Now, in order for these electrons to be release from the metal, they have to perform some work.
Thus, the energy of the absorbed photon E ought to be greater than this ‘work function’ W of the metal, where the work function represents the minimal energy, or threshold energy, needed to liberate an electron from a specific metal.
Now since the energy of the photon is directly proportional to the frequency of the light, the threshold energy corresponds to the threshold frequency f0.
The relationship between energy and frequency is given by this equation, where ‘h’ is the Plank’s constant. The same equation can also be used to calculate the threshold frequency.
For example, the work function of zinc is 4.3 electron-volts. This means the threshold frequency for photoelectric effect to occur in zinc will be 10^15 Hertz, corresponding to a threshold wavelength Λ0 of 300 nanometers. Such a short wavelength corresponds to UV light
Having reviewed the principles behind photoelectric effect, let us now go through the step-by-step protocol to demonstrate this effect through a simple experiment.
Obtain all the necessary instruments and materials for the experiment namely, an electroscope, a zinc metal plate, a piece of sandpaper, a UV source which has a wavelength component below 300nm, a regular lamp providing visible light, an acrylic rod, a piece of fur, and a pair of UV protective eyeglasses.
First, using the sandpaper, polish the zinc metal plate’s surface. This removes the zinc oxide on the metal surface and makes it easier for electron transfer. Place the zinc plate on the metal plate of the electroscope. Make sure that the zinc plate is in direct contact with the electroscope.
Next, rub the rod with the piece of fur five to six times, to make the rod negatively charged. Bring the rod close to the zinc plate making sure not to bring them in contact with each other.
Using the other hand, touch the zinc plate briefly, to positively charge the zinc plate through induction. The needle of the electroscope should deflect to indicate that the metal plate and all the parts in the electroscope connected to it, are charged.
Next, turn on the visible lamp and bring it close to the electroscope and shine its light on the zinc plate. Observe the response of the electroscope.
Now, turn off the regular lamp and put on the UV protective eyewear. Remove the glass plate and turn on the lamp to obtain a UV light source and bring it close to the electroscope. Shine the UV light on the zinc metal. Observe the response of the electroscope. Then turn off the UV light.
Now, rub the rod again with the fur five to six times, to make the rod negatively charged. Bring the rod in direct contact with the zinc plate.
This will result in a deflection of the needle of the electroscope due to the transfer of some negative charges onto the zinc plate. Put away the rod and ensure not to touch the zinc metal plate with your hand or any other object.
Next, turn on the visible lamp and bring it close to the electroscope and shine its light on the zinc plate. Observe the response of the electroscope.
Put on the UV protective eyewear. Remove the glass plate and turn on the UV light and bring it close to the electroscope. Shine the UV light on the zinc metal. Observe the response of the electroscope. Then turn off the UV light.
Let us now review and interpret the results of these experiments.
In the first half of the experiment where the charged rod and the zinc plate are not in direct contact with each other, the needle remains deflected for both the regular lamp and for UV light illumination, indicating the zinc plate remains charged.
This occurs because the zinc plate, which has already lost some electrons to become positively charged, further loses some photoelectrons when the UV light is shone on it. This only makes the zinc plate slightly more positively charged, deflecting the electroscope needle a little bit more.
On the other hand, when the charged rod and the zinc plate are made to come in contact with each other, we observe that using the regular lamplight has no effect on the electroscope. However, the use of the UV lamp results in the needle of the electroscope to collapse and return to the uncharged position with no deflection
This occurs because only UV light photons have enough energy that is above the work function of zinc, to eject photoelectrons. This discharges the zinc plate that was previously negatively charged.
As in the previous case, visible light does not have enough energy to excite photoelectrons, due to which the zinc plate does not discharge.
Photoelectronics has been studied for many decades now and has led to the development of new fields of study and multiple applications.
The photoelectric effect has been used to make various optoelectronic devices that have varied practical applications. One example of an optoelectronic device is the photosensitive electrical switch.
Here, the blocking or unblocking of a light beam shining on a metal turns OFF or ON an electrical current due to the absence or presence of photoelectrons.
Night vision devices or NVDs also use the principles of the photoelectric effect to allow images to be produced in levels of light approaching total darkness. Briefly, photons hitting a thin film of alkali metal or semiconductor material within the device cause the ejection of photoelectrons due to the photoelectric effect.
These electrons are accelerated by an electrostatic field and multiplied through secondary emissions to intensify the original signal. The multiplied electrons are then made to strike a phosphor-coated screen, converting the electrons back into photons, thus forming an image.
You’ve just watched JoVE’s introduction to the Photoelectric effect. You should now understand the basic concepts of the photoelectric effect and also understand why charged metals can be discharged only using light of a specific frequency. In addition, this video demonstrated a simple experiment to visualize the photoelectric effect using a charged zinc metal plate exposed to visible light and UV light. Thanks for watching!
