Stimulated emission

The concept of stimulated emission was first put forward by Albert Einstein in 1917.

Let us consider two energy levels E₁ and E₂ in a material. Let us assume that the atom is initially at energy level E₂. Also, consider that an electromagnetic wave of frequency v is incident on a given material. This wave has the same frequency as that of the specific material. Therefore, there is a high degree of probability that this wave will force the atom to undergo a change from energy level E₂ to energy level E₁. In this case, the energy difference E₂ - E₁ is received in the form of an electromagnetic wave which adds to the incident wave. Here, it is to be noted that the atom is already in the excited state E₂. Before it could come to the ground state, due to the spontaneous emission process, if it is irradiated with a photon, whose energy is exactly equal to E₂ - E₁. The incident photon will stimulate the excited atom to emit one photon of exactly the same energy, as that of the incident photon. Thus, two photons will be emitted in this process. This phenomenon is known as Stimulated Emission. 

Note: The most remarkable feature of the stimulated emission is both the emitted photons will have the same frequency, phase direction and polarization, as that of the incident photon. So, in this process, we give the input of one incident photon and obtain the two photons identical in all respects as an output. Thus, amplification of radiation takes place by the stimulated process. 

You can also look at the difference between spontaneous and stipulated emission. 

Spontaneous emission

Let us consider two energy levels E₁ and E₂ in a material. For convenience, E₁ is taken to be the ground level. E₂ is greater than E₁. ( E₂ > E₁ ) The two levels of energy under consideration can be any two of the unlimited numbers of levels of energy that can be possessed by any material. 

E₁ = Energy level 1 ( Ground state )

E₂ = Energy level 2 ( Excite state ) 

h = Plank's constant

v = Frequency of radiated energy 

When the particle or atom of the material is excited, it can remain in the excited state for a limited time known as a lifetime. The lifetime of the excited hydrogen atoms is of the order of 10^-8 sec. Usually, the number of excited particles in a system is smaller than the non-excited particles. The time during which a particle can exist in the ground state is unlimited. The particle at the excitement level is more unstable as compared to that at the ground level. Naturally, there is a tendency for the particle to come back to ground level. When it comes back to ground level, the particle releases some amount of energy. When this energy is released in the form of electromagnetic waves, we call it spontaneous emission of energy. The frequency of the radiated wave is given by :

v = ( E₂ - E₁ / h ) 

OR 

hv = ( E₂ - E₁ ) 

Note: Radiative emission is just one of the two possible ways for the atom to decay. The decay can also occur in a non-radiative manner. In the non-radiative transition, the difference in energy levels E₂ and E₁ can be experienced in regions other than electromagnetic. 

You can also look at the difference between spontaneous and stipulated emission. 

What is polorization

Light is electromagnetic in nature. The light is transverse in nature. 

The vibration of the electric field and magnetic field are perpendicular to each other and they are also perpendicular to the direction of propagation of light.

The light in which electric field has the freedom to vibrate in all direction which is perpendicular to the propagation of light is known as unpolarised light.

Unpolorised means it has the freedom and it does not have any restrictions.

The light in which the electric field can vibrate only in one direction in another word there is restriction of vibration of an electric field such light is known as polarised light.
The phenomenon in which unpolarised light is converted into polarized light is known as polarization.

There are different method unpolarised light converted into polarised light :

• Using polarising materials.
• By reflection of light.
• By scattering of light. 
• By doable refraction of light.

What is diffraction

Diffraction :

Diffraction is the bonding of light around an edge of the obstacle such as the edge of the slit we can diffraction of light through the crack between two fingers at the distant source.

Diffraction of any kind of waves depends upon the wavelength and the size of the obstacle. 

For significant diffraction, the ratio of λ/d should be 1. 

There are two types of diffraction :

1. Fresnel diffraction
2. Fraunhofer diffraction

Optical wavelength

What is Optical Wavelength?

When there is a medium which has refractive index µ we have to use optical wavelength λ_µ.

The relation between optical wavelength and geometrical wavelength is given by 

λ_µ = λ / µ 

λ_µ = Optical wavelength

λ = Geometrical wavelength

µ = Refractive index of medium

Electromagnetic spectrum definition

The electromagnetic waves can be characterised by the parameters like wavelength, frequency, phase and state of polarization.

Wavelength :
The distance between two-crust or two throughs is known as wavelength. 
Wavelength denoted by λ.

Frequency : 
Frequency is the quantity that represents a number of oscillations that particle carries out in unit time.
Frequency denoted by ν.

Wavelength (λ) = c / Frequency (ν)

Where c = velocity of electromagnetic wave

Wavelength × Frequency = Velocity of the wave

The wavelength of the electromagnetic wave varies from 10^-12 meters to 10⁴ meters.

Phase :

The electromagnetic wave can be represented by a sine or cosine function.

E = E₀ Sin (wt + Φ )

Where E = Position of a wave at time t

E₀ = Maximum displacement of Amplitude

(wt + Φ ) = Phase of the wave

Φ = Initial phase or phase difference

Phase difference :

The difference between the phase if the two waves or phase of two position of a single wave is known as phase difference.

Wave optics Introduction

In a one-word optics can define that science of light.
Light has dual nature sometimes it behaves like a wave and sometimes it behaves like a particle.
In other word deal with optics is the branch of physics dealing with the study of optical phenomena.
The phenomena like reflection and refraction are explained by corpuscular theory. but the phenomena like interference, diffraction and polarisation can not explain the particle nature of light.
In optics, we consider the light as a wave.

We have also seen terms in optics are following below :

• Light 
• Electromagnetic Spectrum
• Wavelength
• Frequency
• Phase
• Phase difference
• Coherence
• Interference 

Coherence definition in physics

Coherence :

If the phase difference between two waves or phase difference between two positions of the single wave remains constant, such waves are known as coherent waves and phenomenon is called coherence.

If the phase difference between two sources changes then sources are known as incoherent. Sunlight, incandescent lamp. tube light etc is an example of the incoherent source.

There are two methods by which we can produce a coherent source :

• Division of wavefront
• Division of amplitude

Types of coherence :

• Temporal coherence
• Spatial coherence

Measurement of coherence :

The coherence of any source can be measured by the visibility of contrast of the fringe system produced by the source. It is given by 

V = I_max - I_min / I_max + I_min 

Where I_max = Maximum Intensity of bright fringe

I_min = Minimum Intensity of dark fringe

Ruby Definition

Ruby :

Ruby is synthetic aluminium oxide ( Al₂O₃ ) with 0.05 weight of chromium oxide ( Cr₂O₃ ) added to it.

It is the chromium that gives the ruby its characteristics red colour.

Application of laser

The main characteristics of laser radiation are following below.

• Coherence
• Very narrow bandwidth
• High directionality
• Extreme brightness

With the above characteristics, the lasers have a wide range of application in different disciplines such as physics, chemistry, medicine, engineering etc. 

Application of laser : 

• The laser is a coherent source, measurement of distances based on interferromagnetic techniques is made much simpler.
• The large distance can be also measured by laser. 
• The time taken for a laser pulse to travel from the laser to the target and back again is measured. Using such a method, the distance between earth and moon have been determined to an accuracy of +-0.15 m.
• The laser beam is highly intense hence it can be used in application like Laser beam welding, Laser cutting of material.
• The laser also is suitable for machining and drilling holes.
• The most successful therapeutic application of the laser has been in eye-surgery for the detached retina.
• Lasers also can serve as war-weapon for military purpose.
• The laser can be used for investigating the structure of molecules.
• Lasers also being employed for separating the various isotopes of an element.
• The narrowness of bandwidth of lasers, the strongest capacity for information in computers is improved.
• Due to a narrow bandwidth, lasers are used in microwave communication. so in the field of communications, laser offers unusual advantages.
• Lasers have also been used for the treatment of dental decay, the destruction of malignant tumours and the treatment of skin diseases.
• Genetic research using laser is quite popular.
• The IBM corporation is trying to transmit an entire memory bank from one computer to another by using a laser beam.