Question

Please Solve: Using the Concept of Higher Level of Physics

classmate Date Page 10 Write down the characteristics and application of Laser @ Write down the applications of optical fiber in medicine and industry. I Give 5 examples of direct and indirect semiconductors. @ what is compound semi-conductors? Give 4 examples of each type. o prove that: N(E) & e L 1 An unknown semi-conductor has Eg=1. lev and NC= N it is doped with 10 cm doners, where the donar lend is o dev below Ec. Given that Ef us 0.25 eV below Ec, calculate ni and the concentration of elections and holes in the semi conductor at 300k? DA A(si) sample is doped with 10 boson atoms/cm . What is the electrons I concentration no at 300K? What is the resistivity? ® A Ge) sample is doped with 3x18 sb atomslem using the requirement of space Change neutrolity calculate the electron concentration to at 300 K?

Answer 1

1. A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for, Light Amplification by Stimulated Emission of Radiation.

Characteristics of Laser:
Laser light has four unique characteristics that differentiate it from ordinary light.
These are:
Coherence
Directionality
Monochromatic
High intensity

Coherence

We know that visible light is emitted when excited electrons (electrons in higher energy level) jumped into the lower energy level (ground state). The process of electrons moving from higher energy level to lower energy level or lower energy level to higher energy level is called electron transition.

In ordinary light sources (lamp, sodium lamp and torchlight), the electron transition occurs naturally. In other words, electron transition in ordinary light sources is random in time. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colours. Hence, the light waves of ordinary light sources have many wavelengths. Therefore, photons emitted by an ordinary light source are out of phase.

In laser, the electron transition occurs artificially. In other words, in laser, electron transition occurs in a specific time. All the photons emitted in laser have the same energy, frequency, or wavelength. Hence, the light waves of laser light have a single wavelength or colour. Therefore, the wavelengths of the laser light are in phase in space and time. In laser, a technique called stimulated emission is used to produce light.

Thus, light generated by the laser is highly coherent. Because of this coherence, a large amount of power can be concentrated in a narrow space.

Directionality

In conventional light sources (lamp, sodium lamp and torchlight), photons will travel in a random direction. Therefore, these light sources emit light in all directions.

On the other hand, in laser, all photons will travel in the same direction. Therefore, the laser emits light only in one direction. This is called the directionality of laser light. The width of a laser beam is extremely narrow. Hence, a laser beam can travel to long distances without spreading.

If an ordinary light travels a distance of 2 km, it spreads to about 2 km in diameter. On the other hand, if a laser light travels a distance of 2 km, it spreads to a diameter less than 2 cm.

Monochromatic

Monochromatic light means a light containing a single colour or wavelength. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colours. Hence, the light waves of ordinary light sources have many wavelengths or colours. Therefore, ordinary light is a mixture of waves having different frequencies or wavelengths.

On the other hand, in laser, all the emitted photons have the same energy, frequency, or wavelength. Hence, the light waves of laser have a single wavelength or colour. Therefore, laser light covers a very narrow range of frequencies or wavelengths.

High Intensity

You know that the intensity of a wave is the energy per unit time flowing through a unit normal area. In an ordinary light source, the light spreads out uniformly in all directions.

If you look at a 100 Watt lamp filament from a distance of 30 cm, the power entering your eye is less than 1/1000 of a watt. In laser, the light spreads in a small region of space and a small wavelength range. Hence, laser light has greater intensity when compared to the ordinary light. If you look directly along the beam from a laser (caution: don’t do it), then all the power in the laser would enter your eye. Thus, even a 1 Watt laser would appear many thousand times more intense than 100 Watt ordinary lamp.

Thus, these four properties of laser beam enable us to cut a huge block of steel by melting. They are also used for recording and reproducing large information on a compact disc (CD).

LASER Applications
Electronic Communication
Light Detection and Ranging(LIDAR)
Medical Industry
Weapons
Electronic Devices and Gadgets

2.Applications of Optical fibre in medicine and industry:
Medicine:
Holmium and Thulium Lasers Make Keyhole Surgery Possible
Laser Radiation at 2 µm for State-of-the-Art Surgery
Destroying Kidney Stones with Laser Light
Stone Destruction Mechanism
Laser Technology in Respiratory Medicine etc.,

Inducstries:
Computers and IT
Defense and Military Applications
Government
Inspections and Imaging etc.,

3. Examples of direct and indirect semiconductors:

Best example of direct band gap semiconductors are GaAs InAs, InSb GaN InN ZnO CdSe ZnS
GaAs emits light in Infra-red region .
Best examples of indirect band gap semiconductors are Si , Ge ,C (diamond) ,GaP.


4.A compound semiconductor is a semiconductor composed of elements from two or more different groups of the periodic table.For example -one element from column III, and one from column V, of the Periodic Table -- the so-called compound III-V semiconductors, such as GaAs, InP and GaN.

In the past, compound semiconductors were not used in the widespread commercial applications and high production volumes typical for silicon. These crystals are more difficult to grow than silicon. The number of defects in the crystal is higher, and the cost of making the crystal is higher. Compound semiconductors also tend to be more fragile. All of these factors limited the growth of compound semiconductors for commercial use.

In recent years, however, the cost of manufacturing compound semiconductors has come down. It is still much higher than silicon, but at the same time, the special properties of these crystals have become more important for certain applications. Because of their fundamental material properties, compound semiconductors can do things that simply aren't possible with silicon.

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