Electron Emission and Photoelectric Effect

In this article, we are going to study about emission of electrons from metal surfaces and a related phenomenon called photoelectric effect.

Table of Contents

• Electron Emission
• Photoelectric Effect
• Photoelectric cell

Electron Emission

We know that, metals are good conductors of electricity. That’s because metals have free electrons in the outer orbit. Metals can conduct electricity as these negatively charged electrons can free themselves from atoms and flow freely in the metal.

However, it’s not easy for these electrons to come out of the metal. That’s because, as soon as an electron on the metal surface leaves its atom, the surface acquires a positive charge and pulls the electron back to the metal.

However, if the electron has enough energy, it may escape. This is akin to how steam leaves the boiling water surface.

Work Function

Work Function of a metal is the minimum energy required by an electron to escape from that metal surface. We use eV (electron volt) to measure it.

To be more precise, Work Function of a metal is the minimum energy required to eject one photoelectron from that metal surface.

Work Function varies from one metal to another, as it depends on the properties of the metal. It also depends on the nature of the metal surface.

eV (electron volt)

eV (electron volt) is an unit of energy that is often used in atomic and nuclear physics.

1 eV = 1.602 × 10^-19 J

1 eV is basically the energy acquired by an electron if it’s accelerated by a potential difference of 1 volt.

Note

• The work function of platinum is the highest.
• The work function of caesium is the lowest.

But how can an electron acquire this minimum required energy and escape from the metal surface?

Energy Sources that can induce Electron Emission

The free electrons in metals can acquire the minimum energy required for emission from the metal surface (i.e. work function) through various physical processes. Some of these have been listed below:

• Thermionic emission: Just like water molecules may escape the water surface if we heat the water, we can do so in case of metals too. If a metal is heated, and sufficient thermal energy is imparted to the free electrons in it, they may emit out of the metal surface.
• Field Emission: Electrons can also be pulled out of a metal surface if a very strong electric field (of the order of 10⁸ V/m) is applied to the metal. This is what happens in a spark plug.
• Photo-electric emission: If light of a certain suitable frequency is illuminated on a metal surface, it may lead to electron emission from its surface. Such electrons are called photoelectrons as they are generated by light/photo.

Now, let’s study about Photoelectric Effect in much more detail.

Photoelectric Effect

As we studied earlier, photoelectric effect is the phenomenon of emission of electrons from a metal surface when light of suitable frequency is incident on it. Such emitted electrons are called photoelectrons.

Many scientists helped us in understanding this phenomenon.

• This phenomenon was discovered by Heinrich Hertz in 1887.
• Thereafter, during 1886-1902 Wilhelm Hallwachs and Philipp Lenard studied it in much more detail.
• It was Einstein who explained the mechanism behind this phenomenon on the basis of Maxwell-Planck quantum theory.
• In 1916, it was proved experimentally by Millikan.

Mechanism behind Photoelectric Effect

Mechanism behind Photoelectric Effect was theorized by Einstein, and then later on experimentally proved by Millikan.

• Radiation/Light is incident on the metal surface.
• Electrons in the metal absorb the quantum of energy of radiation.
• If the quantum of energy of radiation being absorbed by an electron exceeds the work function of that metal (i.e. the minimum energy needed for the electron to escape from the metal surface), it may escape from the metal surface with maximum kinetic energy. This is related to the concept of threshold frequency.

Threshold frequency and Threshold wavelength

We already know that, for any particular metal or photosensitive material, the quantum of energy of the incident radiation must be over the work function of that metal.

We also know that, the the quantum of energy of the incident radiation (or light) is dependent on the wavelength/frequency of the radiation/light. More the frequency of light, more energetic will be the photons.

Threshold frequency for a metal is the minimum cut-off frequency of the incident radiation, below which no emission of photoelectrons takes place. So basically, threshold frequency is nothing but the frequency of the work function.

It doesn’t matter how intense the light is – because energy of photons is dependent on the frequency of light, and not on its intensity.

Above the threshold frequency, the maximum kinetic energy of the emitted photoelectrons increases linearly with the frequency of the incident radiation. This is also independent of its intensity.

Threshold wavelength is the maximum cut-off wavelength of incident radiation/light that can eject photoelectron from a metal surface. It is denoted by λ_max.

Characteristics of Photoelectric Effect

Property 1: Intensity of incident radiation

Number of photoelectrons emitted per second α Intensity of incident radiation

Intensity of radiation is the number of photons passing through an area per second (or incident on a metal surface per second).

More the intensity of the incident radiation, the more will be the number of photoelectrons emitted per second.

However, keep in mind that the maximum kinetic energy of the emitted electrons does not depend on the intensity of the incident radiation. It rather depends on the energy of the incident photons.

Property 2: Energy of incident radiation

Maximum kinetic energy of the emitted photoelectrons depends on the energy of the incident photons and the metal (i.e. emitter plate material).

In other words, as the kind of incident light source (i.e. as wavelength/frequency of the light) will change, the maximum kinetic energy of the emitted photoelectrons will change.

It does not depend on the intensity of the incident radiation.

Property 3: Instantaneous Process

Photoelectric emission happens instantaneously, i.e. there is no apparent time lag (∼10^-9 s or less) between radiation being incident on the metal surface and photoelectric electrons being emitted.

This holds true even when the incident radiation is made exceedingly dim.

Property 4: Nature of Metal and Light

Photoelectric emission varies from one metal to another, and from one kind of light to another.

• How a material will respond to a certain light will vary from one photosensitive material to another. For example, selenium is much more sensitive to light than zinc or copper.
• If different kinds of light (i.e. lights of different wavelengths/frequencies) are incident on the same metal, it will respond differently. For example, we get to see photoelectric effect in copper if ultraviolet light is incident on it. But we do not get to see photoelectric effect in copper if green or red light is incident on it.

Photoelectric cell

Photoelectric cell is a device that converts light energy into electrical energy. It is based on the phenomenon of photoelectric effect.

Stopping potential or Cut-off potential: It is the negative potential given to the anode of a photoelectric cell for which photoelectric current becomes zero. It is denoted by V₀.

Applications of Photoelectric cells

Photoelectric cells are used in various instruments. Some of them have been listed below.

• Vapour Iamps photoelectric cells are used in the street light. They convert the incident sunlight into electric energy and store it. Such light can automatically get switched on during the night, and switched off during the day.
• Photoelectric cells are used in space satellites too. They get charged when sunlight falls on them – that is they convert light energy into electrical energy. At night the stored electricity is used up.
• Photoelectric cells are used in automatic doors, that slide open on their own as a person approaches them. We often encounter such doors in multiplexes, state-of-the-art modern offices, etc.
• Photoelectric cells are used in strong rooms of banking institutions to identify any unwanted person.
• Photoelectric cells are used for reproduction of sound in cinema and TV.