Particle Nature Of Light – Photocell

Studies over the years have shown that light has a dual nature. It shows duality as it has both wave nature and particle nature. The photoelectric effect is an important aspect of the particle nature of light. 

 The photoelectric effect is the primary principle behind the workings of a photocell. They come in use in various ways in our day-to-day lives. From street lamps to security alarms, they can be found in many places. In this article, we will give you an overview of what is photoelectric effect and also what is a photocell. We shall also list down a few common applications of a photocell. So read on to find out.

Dual nature of light:

Albert Einstein in 1905 proposed that light can be found as “quanta” of energy that imitates or behaves like particles instead of waves. Here, quanta are regarded as small packets of energy. This gives rise to photons. A photon can be described as a minuscule particle of electromagnetic radiation whose mass is zero and carries a quantum of energy. 

A light ray is made up of such photons. The energy contained by the photons in a light ray can be measured through the equation given as ,

E=h; where h is the Planck’s Constant and is the frequency of light. 

A very important experiment that demonstrated the particle nature of light waves was the photoelectric effect. 

Photoelectric Effect :

The photoelectric effect can be described as a phenomenon that leads to electrons being ejected from a metallic surface when a light ray falls on it. This phenomenon is also known as photoemission. This phenomenon can only occur for certain metallic surfaces. Such metallic surfaces are known as Photosensitive metals. 

Classic physicists believed that the number of photons emitted and the kinetic energy carried by these photons depend only upon the intensity of the light ray and not on its frequency. 

However, it was found that each metallic surface has a particular threshold frequency. If the frequency of the light is below the threshold frequency of the metallic surface, then the electrons are not ejected, irrespective of how much the intensity is increased. Only when the frequency of the light is equal to the threshold frequency, then the electrons are ejected from the metallic surface. 

The minimum frequency that is needed for electrons to be ejected from a metallic surface is known as the threshold frequency. 

As we know, frequency and wavelength are inversely proportional to each other. So when the wavelength is low, the frequency is high and vice-versa. So for example, Red has the highest wavelength thus it has the lowest frequency.

Thus, if a red light is shone on a surface and the frequency doesn’t reach the threshold frequency, electrons will not be ejected from the metallic surface irrespective of the intensity or the time for which it is shown. 

On the other hand, if a green light is shone on a metallic surface, then having a higher frequency, it will be able to eject electrons from the metal. 

Equation for photoelectric effect: 

The minimum energy required by a photon for photoemission to occur is given by the equation:

E=h; where h is the planck’s constant and is the frequency. 

Since h is a constant, therefore the minimum energy only depends upon the frequency of the light. If the frequency of the light is equal to the threshold frequency, then it will be able to eject electrons from the metallic surface. 

If the frequency of the light is more than the threshold energy, then the ejected electrons will move at a faster speed. If the intensity of the light is increased, that has a frequency above the threshold frequency, then the speed of the electrons won’t increase, but the number of electrons being ejected increases.

This phenomenon of the photoelectric effect is used in the making of a photocell or photoelectric cells.

What is a photocell ?

A photoelectric cell, more popularly known as a photocell, is a light-sensitive device that is the best example of photoelectric effect as it is made based on that effect as principle. Due to the photoelectric effect, when a light ray falls on a photocell, then it turns the light energy into electrical energy. As electrons are emitted, these electrons start flowing, leading to the creation of electricity. 

A photocell is found in the form of a tube that contains two electrodes – namely the cathode and the anode. These two electrodes are separated by the light-sensitive semiconductor material. 

The photocell is a completely evacuated glass tube or it can also be in the form of a quartz bulb. The cathode is made up of a photosensitive material, so the light enters through a quartz window and falls on the cathode. 

Instead of having the two electrodes, sometimes a photocell also has an emitter in the form of a photosensitive metal plate and a collector in the form of a wire loop. This is then connected to a circuit and an ammeter. 

When the light of a frequency above the threshold frequency falls on the photosensitive material, then photoelectrons are emitted. These photoelectrons are then collected at the collector. The flow of these electrons raises the current, which is then recorded on the ammeter connected to the circuit. 

Applications of a Photocell:

A photocell has a myriad of applications. A few examples are:

1. Photocells are used in burglar alarms.
2. It is also used in fire alarms .
3. They are also used in dusk-to-dawn lights. 
4. They are used in sensors.

Conclusion

In this article, we learned how the photoemission phenomenon occurs. The photoelectric effect can simply be defined as the phenomenon which leads to the ejection of electrons from a metallic surface due to certain conditions. This is in turn used in converting light energy into electrical energy, as there is a flow of electrons involved. 

We also got an overview of what a photocell is and how they work. Photocells also have a wide range of applications, from sensors to security alarms.