Modern Physics Concept Page - 1

Formula

Energy of a photon

Photons of frequency ν$\nu$ and wavelength λ$\lambda$ will have energy, E=hν=hcλ$E=h\nu ={\displaystyle \frac{hc}{\lambda }}$

Example

Example of energy of photon

Example: Consider a parallel beam of light of wavelength 600$600$ nm and intensity 100$100$ Wm−2$W{m}^{-2}$. Then energy of each photon is :
E=hcλ=(4.14×10−15eVs)×(3×108ms−1)600×10−9=2.07eV$E={\displaystyle \frac{hc}{\lambda }}={\displaystyle \frac{(4.14\times {10}^{-15}eVs)\times (3\times {10}^{8}m{s}^{-}1)}{600\times {10}^{-9}}}=2.07eV$ 

Definition

Photoelectron

When light of suitable frequency shines upon a metal surface electrons are emitted from the surface. These electrons are called photoelectrons.

Example

Application of photoelectric effect and Laser

Before discussing about the uses of photoelectric effect let us try to recall our experiences in modern physics. You might have come across  such experiences.
A model of automatic street light. In this model when there is sunlight street light will  be off. When sun sets, street light will be on. Such models can be design by you also using a photocell as a switch.
Laser in modern life has brought about revolutionary  changes in the medical field and industrial field. There are various types of laser available in the market,. Some of them are
1. Solid-state laser: Its have lasing material distributed in a solid matrix such as neodynium yittrium-aluminium garnet.
2. Gas laser: Helium and helium-neon-HeNe, are the most common gas laser. It has a primary output of visible red light.
3. Excimer laser: It use reactive gases, such as chlorine and fluorine, mixed with inert gases such  as argon, krypton or xenon.
4. Dye laser : It use complex organic dyes , such as Rhoda min 6G, in liquid solution or suspension as lasing media.
5. Semiconductor laser: It sometimes called diode laser, are not solid-state laser. These electronic devices are generally very small and use low power. 

Definition

Introduction to electron emission

Metals have free electrons. These electrons are responsible for their conductivity. But these free electrons cannot escape out of the metal surface. There is a reason why. If an electron come out of metal surface, the metal surface acquires a positive charge and pulls the elctron back to the metal. Hence the electrons are held inside the metal due to the attractive force of ions. Hence the metals can come out of the metal surface when we provide enough energy to the electron.

Definition

Different methods of electron emission

• Thermionic emission: When we provide enough heat energy to the metal, the free electrons get extra energy. the electrons can come out of the metal using this extra energy.
• Field emission: By applying a strong electric field ( of the order of 108Vm−1${10}^{8}V{m}^{-1}$) to a metal, electrons can be pulled out of the metal.
• Photo-electric emission: Light of different frequencies have different energies. More the frequency more will be the energy of the light. Hence when a light with suitable energy is incident on a metal surface, the electrons are emitted from the metal surface. These electrons are called photo-electrons.

Example

Work function of common metals

The minimum amount of energy required to emit electrons from a metal surface is called the work function. It is characteristics of a metal. Work function for some metals (in evs) are as follows:

Platinum	6.2
Tungsten	4.52
Chromium	4.37
Zinc	4.24
Sodium	2.3

From above table it's evident that the alkali metals have low work function.

Definition

Work function

We know that electrons may emit from a metal surface when light is incident on the metal. But a minimum energy is required for this process. This minimum energy is different for different metals.
This minimum energy is called work function of the metal.

Definition

Unit of work function

Work function is the minimum energy required for a metal to emit electrons. Hence it has unit of energy. It may be Joules, or eV (electron volts). Generally the work function of the metals are of the order of electron volts. Hence they are generally represented in eVs.

Definition

Thermionic Emmision

The emission of electrons from a metal surface when heat energy is imparted to it is called the thermionic emission. Thermionic emission is the thermally induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the work function of the material. After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region. But if the emitter is connected to a battery, the charge left behind is neutralized by charge supplied by the battery as the emitted charge carriers move away from the emitter, and finally the emitter will be in the same state as it was before emission.

Definition

Factors affecting the rate of thermionic emission

The number of electrons emitted per second from a surface depends on the following three factors:
1. The nature of the metal surface: Lower the work function of the metal, greater is the rate of emission of electrons from its surface.
2. The temperature of the surface: Higher the temperature of the surface, greater is the rate of emission of electrons from the surface.
3. The surface area of the metal: Larger the surface area of the metal emitting the electrons, greater is the rate of emission of electrons.

Definition

Holography

A hologram is a physical structure that diffracts light into an image. The term 'hologram' can refer to both the encoded material and the resulting image. A holographic image can be seen by looking into an illuminated holographic print or by shining a laser through a hologram and projecting the image onto a screen.

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