 Namaste. I, Dr. Mrs. Preeti Sunil Joshi, Assistant Professor from Department of Humanities and Sciences, Balchan Institute of Technology, Solapur. I welcome you in the session in which we are going to study the different characteristic properties of laser. The learning outcomes of this session are, by the end of this session the students will be able to know the differences between ordinary light and laser light and also understand the characteristic properties of laser. The contents include characteristic properties of laser. Laser and light are the two terms that are frequently used in physics. Generally light and lasers are considered as travelling photons. Laser is acronym for light amplification through stimulated emission of radiation. Students in the previous videos we have seen the three different processes that are required for the operation of laser. So we know what is stimulated emission. Let us revise some points. Stimulated emission is called stimulated as it occurs due to the stimulated photon. Due to the external photon, this type of emission takes place. Then the meaning of radiation is the transfer of energy that is from higher level to lower level or it can be from lower level to higher level. But here as the emission takes place, the energy transfer is from higher level to lower level. So in stimulated emission due to the incident photon, emission takes place and that results in the amplification of light. For one incident photon, we are getting two photons which are exactly identical with the incident photon in phase, direction, intensity in all its aspects. It is identical with the incident photon. However, laser is not a simple amplifier of light but it is a generator of light. Laser is an artificial source that differs vastly from the traditional light sources. Students, you might have seen the laser light in different places. So please pause the video and try to write down the differences that you have observed between ordinary light and laser light. So laser light is differentiated from ordinary light by four characteristics. High directionality, high monochromaticity, high coherence and high intensity. These different properties are used for different applications. So let us first know these properties one by one in detail. The first important property of laser is directionality. When you switch on a torch, the light is emitted in all directions. Similarly, the light from all conventional sources like the electric bulb, lamp dispenses in all possible directions. So this property of light to spread in all directions is called divergence. However, contrary to conventional sources of light, lasers emit light only in one direction. This property of laser to give off light in only one direction is called directionality. The photons are emitted when an excited electron comes back to the ground state. All minions of photon are emitted in the same direction and appear in the form of a beam of laser. Consider a powerful search light in a lighthouse that is used in a dark night. The beam of this light can reach out to as far as 1 kilometer from the place of its source and spread to a diameter of about 1 kilometer in front of it. The laser on the other hand has the ability to focus the same amount of light into a diameter of just 1 centimeter as shown in the figure. This is the power of directionality of laser beams. It is highly concentrated light and this property of laser that is directionality is very useful for focusing light in the required direction during many applications. Now let us know what is monochromaticity. This is another important property of laser light. Monochromaticity is the term that refers to one color which has same wavelength and with high intensity of light. Light light is a combination of all colors. It consists of many frequencies and thus corresponding spreadings and scatterings. Now consider a sodium lamp that emits light in a single color. The sodium lamp is most common example of monochromatic light as it emits light of only one wavelength that is measured in lambda. The wavelength is 5893 angstrom in it and its intensity is also very high. Laser in general generates the light in a very narrow band around a single central wavelength. The generation of laser is such that the laser cavity forms a resonant system and laser oscillation is sustained only at the resonant frequencies of the cavity. This leads to the further narrowing of the laser line width. So the laser light is usually very pure in wavelength and hence we say that the laser has the property of monochromaticity. Actually the degree of monochromaticity can be quantitatively described in terms of wavelength bandwidth or frequency bandwidth. The range of frequency of wavelength of the maximum intensity is called as bandwidth. The narrower in the line width, higher degree of monochromaticity of the laser has and large amount of energy can be concentrated even in small bandwidth due to high monochromaticity of light. Now let us see the third important property of laser that is coherence. We all know that how the light is emitted from the ordinary light source. In an atom when the electrons are in excited state they jump from higher state to ground state that is known as transition. In ordinary light sources the electronic transition takes place randomly but when the light waves are received from ordinary light source their wavelength has no definite shape in space and as well as in time and hence these light sources are said to be as incoherent. On the other hand in case of laser light the electronic transition takes place in an orderly way and their wavelength has consistent phase with time which is called as coherence. In case of laser a large number of identical photons are emitted through stimulated emissions and therefore they will be in phase with each other and hence the resultant light exhibits a high degree of coherence. Now the another property of laser is high intensity. Intensity of a wave is defined as energy per unit time flowing through a unit normal area. The ordinary light spreads uniformly in all directions and forms a spherical wave front around it whereas in laser light energy is concentrated in small regions of space with small wavelength but with greater intensity. Let us consider an example when we see at 100 watts lamp filament from a distance of about 30 centimeter then less than 1000th of a watt of the power enters into our eyes. In case of laser when we see along beam of laser all the power of light enters into our eyes and in this case even 1 watt laser would appear 1000 times more than 100 watts of ordinary lamp. So let us come to the conclusion of this session. So in this session we have seen that laser is an extraordinary light that is emitted under stimulated and amplified conditions so that the beam is characterized by high intensity, specific directionality, high monochromaticity and with high degree of coherence. Now please pause the video and try to solve these multiple choice questions. Check for the correct answers. Thank you.