What is Photoelectric Effect

Introduction to the photoelectric effect When light shines on certain substances, it causes changes in the electrical properties of the substances. This type of photoelectric effect is collectively referred to as the photoelectric effect. The effect of metal surfaces emitting electrons under the action of light irradiation is called photoelectrons. Electrons can only be emitted when the wavelength of light is less than a certain critical value, that is, the limiting wavelength, and the corresponding frequency of light is called the limiting frequency. The critical value depends on the metal material, and the energy of the emitted electrons depends on the wavelength of light and has nothing to do with the intensity of light. This cannot be explained by the wave nature of light. Another point that contradicts the wave nature of light is the instantaneous nature of the photoelectric effect. According to the wave theory, if the incident light is weak, the irradiation time must be longer for the electrons in the metal to accumulate enough energy and fly out of the metal surface. But the fact is that as long as the frequency of light is higher than the limiting frequency of the metal, no matter how strong or weak the light is, the generation of photons is almost instantaneous, not exceeding ten to the negative ninth power of seconds. The correct explanation is that light must be composed of strictly regulated energy units (i.e. photons or light quanta) related to wavelength. This explanation was proposed by Einstein. The photoelectric effect was discovered by German physicist Hertz in 1887 and played a fundamental role in the development of quantum theory. The phenomenon of electrons being released from an object under the irradiation of light is called the photoelectric effect. The photoelectric effect is divided into photoelectron emission, photoconductivity effect and photovoltaic effect. The former phenomenon occurs on the surface of an object and is also called the external photoelectric effect. The latter two phenomena occur inside an object and are called the internal photoelectric effect. In the photoelectric effect, the direction of electron emission is not completely directional, but most of them are emitted perpendicular to the metal surface, which has nothing to do with the direction of light. Light is an electromagnetic wave, but light is an orthogonal electromagnetic field with high-frequency oscillations, and the amplitude is very small, which will not affect the direction of electron emission.

Photoelectric effect

Photoelectric Effect Explanation

Experimental laws of the photoelectric effect.
a. The photoelectron beam emitted by the cathode (metal material that emits photoelectrons) is proportional to the luminous intensity of the irradiation.
b. The initial velocity of the photoelectron when it escapes from the object is related to the frequency of the irradiating light but has nothing to do with the luminous intensity. That is to say, the initial kinetic energy of the photoelectron is only related to the frequency of the irradiating light but has nothing to do with the luminous intensity.
c. Only when the frequency of the light irradiating the object is not less than a certain value can the object emit photoelectrons. This frequency is called the limiting frequency (or cutoff frequency), and the corresponding wavelength λ is called the red limit wavelength. The limiting frequency of different substances and the corresponding red limit wavelength λ are different.

Einstein's equations

hυ=(1/2)mv^2+I+W Where (1/2)mv^2 is the initial kinetic energy of the photoelectrons escaping from the object. There are a large number of free electrons inside the metal, which is a characteristic of the metal. Therefore, for metals, the I term can be ignored, and the Einstein equation becomes hυ=(1/2)mv^2+W. If hυ

Classification of photoelectric effect :

External photoelectric effect and internal photoelectric effect.

External photoelectric effect
The phenomenon that electrons in an object escape from the surface of the object and emit outward under the action of light is called external photoelectric effect.
In 1887, Hertz (M.Hertz) first discovered it in an experiment to prove the wave theory;
in 1902, Lenard also studied it and pointed out that the photoelectric effect is the phenomenon that electrons in metals absorb the energy of incident light and escape from the surface. But it could not be explained according to the theory at that time;
in 1905, Einstein proposed the photon hypothesis.
Einstein's photoelectric effect equation:
1. Whether photoelectrons can be generated depends on whether the energy of the photon is greater than the surface electron escape work function A of the object.
2.  When a certain value is reached, the photocurrent generated is proportional to the light intensity.
3. The escaped photoelectrons have kinetic energy.
Photoelectric devices based on the external photoelectric effect include phototubes and photomultiplier tubes.

Internal photoelectric effect
When light shines on an object, the conductivity of the object changes, or a photoelectric potential is generated. It is divided into photoconductivity effect and photovoltaic effect (photovoltaic effect).
1 Photoconductivity effect
Under the action of light, electrons absorb photon energy and transition from a bonded state to a free state, causing a change in the conductivity of the material.
When light shines on a photoconductor, if the photoconductor is an intrinsic semiconductor material and the light radiation energy is strong enough, the electrons in the valence band of the photoconductor will be excited to the conduction band, increasing the conductivity of the photoconductor.

Optoelectronic devices based on this effect include photoresistors.

Working principle diagram of photoresistor

2. Photovoltaic effect: the phenomenon that an object can generate an electromotive force in a certain direction under the action of light. Devices based on this effect include photocells, photosensitive diodes, and triodes.

① Barrier effect (junction photoelectric effect)
When light irradiates the PN junction, if h≧Eg, the electrons in the valence band will jump to the conduction band, and electron-hole pairs will be generated. Under the action of the electric field in the barrier layer, the electrons will be biased to the outside of the N region, and the holes will be biased to the outside of the P region, making the P region positively charged and the N region negatively charged, forming a photoelectric potential.

② Lateral photoelectric effect
When a semiconductor photoelectric device is illuminated unevenly, electron-hole pairs are generated in the illuminated part, and the carrier concentration is greater than that in the unilluminated part. A carrier concentration gradient appears, causing carrier diffusion. If the electrons diffuse faster than the holes, the illuminated part will be positively charged and the unilluminated part will be negatively charged, thereby generating an electromotive force, which is the lateral photoelectric effect.