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Photoelectric Effect

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Transcript

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!

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