(General Science) PHYSICS - Optics: Power of a Lens, Internal Reflection,Fiber Optics, Lasers and their Application

GENERAL SCIENCE: PHYSICS


Power of a Lens

The degree of convergence or divergence of light rays achieved by as lens is expressed in terms of its power. The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P. the power P of a lens of focal length f is given by P=1/f. SI unit of power of a lens is ‘dioptre’ . it is denoted by the letter D. the power of a convex lens is positive and that of a concave lens is negative.

Refraction and Dispersion of Light through a Prism


A prism ahs two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other. The angle between its two lateral faces is called the angle of the prism, A prism splits the incident white light into a band of colours. The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR). The splitting of light into its component colours is called dispersion. Different colours of light have different refractive indices and hence bend through different angles with respect to the incident ray as they pass through a prism, causing a spectrum of colours . the red light bends the least while the violet the most.

Rainbow: A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun. the water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colours reach the observer’s eye.  

Total Internal Reflection


Light can always pass from one medium to an optically denser medium but it cannot always pass into a rarer medium. If the angel of incidence of light in the denser medium is greater than a particular angle (known as the critical angle for that medium). The light is not at all refracted into the rarer medium but is totally reflected. This is known as total Internal Reflection.

Applications of Total Internal Reflection: When light is incident at one end of fiber. It undergoes repeated total internal reflections and emerges at the other end (Details are covered in the section on fiber Optics)

Mirage: it is usually associated with hot deserts. The air in the desert is hot near the ground and cools rapidly with height. The hotter air is optically less dense. Rays of light from the top of a tree (on the sky) suffer successive bending as they pass through the warmer layers of decreasing density. This results in the gradual increase of the angle of incidence. Eventually, a stage comes when the angle of incidence exceeds the critical angle and, therefore total internal reflection takes place. After this the rays start bending upwards. An observer sees the tree upside down (as well as the actual tree) as if he were seeing the reflection on a surface of water (See fig.) On hot summer days, motorists quite often see similar mirages on the roads.

Scattering of Light


If the molecules of a medium, after absorbing incoming radiations, emit them in all possible directions, the process is called scattering.

Rayleigh has shown that the intensity of scattered light is inversely proportional to fourth power of wavelength (or directly proportional to fourth power of frequency).

Some effects of scattering

  • Tyndall effect: when a beam of light strikes the fine particles of atmosphere or any colloidal solution, the path of the beam becomes visible. This is due to the scattering of light by the colloidal particles and is called Tyndall effect. 
  • Blue Colour of sky: when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes, thus making sky look blue to us.
  • Danger signal lights are red in colour as red is least scattered by fog or smoke therefore, it can be seen in the same colour at a distance.
  • Reddish appearance of sun at sunrise and sunset: light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes (See fig.). Most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths, giving rise to the reddish appearance of the sun. However, light from the Sun overhead travels relatively shorter distance through atmosphere and hence only little scattering occurs at noon.

Some other effect associated with Light

Interference: the superposition of two (or more) waves of the same kind that pass the same point in space at the same time is called interference. If the waves are in the same phase, e.g., crest on crest, their amplitudes combine to produce a strong wave (bright spot). This is called constructive interference. If the waves are out of phase, e.g., if crests of one are superposed on the trough of another, we get destructive interference (dark spot). Thus interference generally produces alternate dark and bright bands on a screen if monochromatic light is used. For white light, coloured bands are obtained.

Beautiful colours seen in soap bubbles and oil films on water are also produced due to the interference of white light reflected by these surfaces.

Diffraction: when a beam of light passes through a narrow slit or an aperture, it spreads out to a certain extent into the region of geometrical shadow. This is called diffraction which in other words is the failure of light to travel in a straight line. Diffraction is a particular case of interference and is due to the wave nature of light. A diffraction grating  is a device used to cause diffraction. Gratings may be prepared by ruling equidistant parallel lines on to a glass ( transmission grating) metal surface (reflecting grating).

The data on a CD are in the form of pits arranged in a spiral. The spiral has its turns so closely wound that a CD acts like a reflection grating. So when a CD is viewed in white light one sees rainbow like colours  due to reflection and diffraction.

Holography : it is the technique of recording and reproducing three dimensional images. A laser beam partly reflected from an object and partly from a mirror produces interference fringes on a photographic plate, which then becomes a hologram. When laser light is transmitted through the hologram, one can see a three-dimensional virtual image of the object.

Fiber Optics


An interesting use of the total internal reflection is in optical fibers, which are fine strands of high quality glass about the diameter of a human hair. When light is incident at one end of the fiber, it undergoes repeated total internal reflections and emerges at the other end without suffering from any transmission losses as shown in the figure.

Basic structure of an optical fiber

The basic structure of an optical fiber consist of three parts; the core, the cladding and the coating or buffer as shown in the figure. The core is a cylindrical rod of dielectric material. Dielectric material conducts no electricity. Light propagates mainly along the core of the fiber. The core is generally made of glass. The core is surrounded by a layer of material called the cladding. The index of refraction of the cladding material is less than that of the core material so that total internal reflection may occur. For extra protection, the cladding is enclosed in an additional layer called the coating or buffer. The coating or buffer is a layer of material used to protect an optical fiber from physical damage. The material used for a buffer is a type of plastic.

Plastic of Optical Fibers

  • Single mode Fibers: they have small cores but tremendous carrying capacity and low intrinsic transmission losses which makes them an ideal transmission medium for long distances.
  • Multi Mode Fibers: they have a larger core than single mode fibers but have a lower capacity in terms of information transmitted and hence are used in systems with short transmission distances.

Advantages of Optical Fibers over conventional copper cables

Less attenuation – it is especially advantageous for long-distance communication, because light propagates through the fiber with little attenuation compared to electrical cables.

Immunity to Electromagnetic Interference – Fiber optics are immune to any Electromagnetic interference since signals are transmitted as light instead of current.

Data security – Magnetic fields and current induction in copper cables let the information on the conductor to be leaked out. There are no radiated magnetic fields around optical fibers; the electromagnetic fields are confined within the fiber. That makes it impossible to tap the signal being transmitted through a fiber.

No Spark Hazards – In some cases, transmitting signals electrically can be extremely dangerous. Most electric potentials create small sparks. Fiber optic cables do not produce sparks since they do not carry current.

Ease of Installation – Increasing transmission capacity of wire cables generally makes them thicker and more rigid. Such thick cable can be difficult to install in existing building where they must go through walls and cable ducts. Fiber cables are easier to install since they are smaller and more flexible.

High Bandwidth Over Long Distances – Fiber optics have a large capacity to carry high speed signals over longer distances without repeaters than other types of cables. As they are thinner than copper cables, so more fibers can be bundled into a given diameter cable, thus increasing the information carrying capacity or bandwidth.

Disadvantages of Optical Fibers

  • Limited physical arc of cable. If bent too much, it will break.
  • It is difficult to spice.
  • Physical vibrations of the cable will show up as signal noise.
  • Loss of light in fiber due to scattering and other effects takes place.

Uses and Applications of Optical Fibers

Telecommunications Industry – In telecommunication systems, information, which can be either in the analog or digital from reaches the transmitter through a coder. The coder converts information into a sequence of pulses (bits). The transmitter is usually a semiconductor laser or LED which is modulated by an information bearing signal and converts electrical signals into light. After passing through the fiber, this light reaches the receiver that converts optical signal into the information bearing signal. This electrical signal after demodulation in decoder produces the audio signal.

Medicine Industry – Bundles of tiny optical fibers are used by doctors to see the inside of a patient’s stomach. Light is piped down some of the fibers to illuminate the inside of the stomach and is reflected back along some other fibers. This procedure is called endoscopy.

Mechanical Imaging – It can be used for inspecting pipes, engines, airplanes etc.

Fiber scope is a flexible fiber optic bundle with eyes piece and lens at the other end used for inspection works to examine components in a tightly packed environment.

Other Uses – Optical fibers can be used for the purposes of illumination, often carrying light from outside to rooms in the interiors of large building. Another important application of optical fibers is in sensors. If a fiber is stretched or squeezed, heated or cooled or subjected to some other change of environment there is usually a small but measurable change in light transmission. Hence, a rather beams from fixed installations within factories to the point of use of the laser light for welding, cutting or drilling.

Lasers


A laser is an optical device that produces an intense beam of coherent monochromatic light. A laser is not a source of energy. It is simply a converter of energy taking advantage of stimulating emission to concentrate a certain fraction of energy (commonly 1%) into radiation of a single frequency, moving in a single direction.
Although Albert Einstein gave the idea of laser (without using this acronym) in 1971, scientists began work on the idea only in 1950. American scientist Gordon Gould suggested the name Laser in 1957. The first working laser was built in 1960 by  the American scientist Therodore Maiman.

Components of Laser
A laser consists of -

  • A gain medium
  • A mechanism to supply energy to it
  • Optical feedback mechanism

The gain medium is a material with properties that allow it to amplify light by stimulated emission. Light of a specific wavelength that passes through the gain medium is amplified (increases in power).

For the gain medium to amplify light, it needs to be supplied with energy. This process is called pumping. The energy is typically supplied as an electrical current, or as light at a different wavelength. Pump light may be provided by a flash lamp or by another laser.

The most common types of laser uses feedback from an optical cavity – a pair of mirrors on either and of the gain medium. Light bounces back and forth between the mirrors, passing through the gain medium and being amplified each time. Typically one of the two mirrors, the output coupler, is partially transparent. Some of the light escapes through this mirror.

Some common applications of lasers

Communication – Communications can be carried in a laser beam directed through space, through atmosphere, or through optical fibers that can bend like cables.

Computers and Electronics

  • Laser are used for recording and storing information including recording of music, motion pictures, computer data etc on a CD in the pattern of tiny pits. A highly focused laser beam allows much more information to be stored on a CD (700MB) by using infra red laser, DVD (4.7 GB) using red laser and Blue ray disc using blue laser, with a storage capacity of 25 GB or even more.
  • Laser can also read and play back the information recorded on a disc. In a CD or DVD player, a laser beam reflects off the pattern of pits s the CD spins and other devices in the player change the reflections into electrical signals and decode them into music.
  • Laser also greatly enhance the speed of computers, so the superconductors use semiconductor laser of Gallium Arsenide.

Barcode scanners – Supermarkets use barcode scanners that use laser to scan the universal barcode to identify products.

Heat treatment – A laser beam’s highly focused energy can produce a great amount of heat and thus can be used for cutting, welding and even for drilling holes.

Holography – It is a method of storing and displaying a 3D image using laser beams usually on a photographic plate or light sensitive material.

Nuclear research – Scientists use laser to produce controlled miniature hydrogen bomb explosion. Laser are used for concentrating huge amount of energy on hydrogen pellets in bringing them up to the thermo- nuclear temperatures. The laser is already playing a concrete role is speeding up the day when we may control without making incision.

Surveying and measurement – Lasers are used to measure distances. An object’s distance can be measured by measuring the time a pulse of laser light takes to reach and reflect back from the object. Laser device used to measure shorter distance are called Laser Range Finders. Surveyors use laser devices to get information needed to makes maps. Laser beams have been used to measure the exact distance between the earth and the moon and to provide information laser lights to measure the speed of vehicles. A laser speed gun takes a large number (say 1000) of reading per second, it can compare the change in distance between readings and calculate the speed of vehicles.

Military applications – Missiles may have laser beam detectors which help in seeking reflected beam and accordingly adjust their flight to hit the spot where the beam is aimed. Under advanced military applications, US is developing  ‘Mobile tactical high energy laser’ (MTHEL) which could be used as field deployable weapons system able to track incoming missiles by radar and destroy them with powerful Deuterium Fluoride Laser. Strategic Defence Initiative of US, also called as STAR WARS, involve space based laser system to destroy incoming Intercontinental Ballistic Missiles. In India, DRDO has made a laser range finder to  be used in the Main Battle Tank ‘Aujun’ to improve its accuracy. ISRO has developed LIDAR (Laser detection and ranging laser) for tracking satellites.

Other Uses – The detection and measuring of pollutants in vehicular exhaust gases is accomplished with lasers. A laser beam is used as a knife to rapidly and accurately cut cloth in garment factories, as a tool for meat inspection, and for finger print detection.

Some practical lasers and their applications


Ruby laser – It is also known as crystal laser, it gives a monochromatic coherent light in pulse.
Helium-neon laser – Unlike the pulse from a ruby laser, a helium-neon laser produces a continuous beam. This is the laser whose red beam is used in the checkout counters of shops and supermarkets to read barcodes.

Chemical laser – Chemical laser are efficient and can be very powerful; one chemical laser in which hydrogen combine to form hydrogen fluoride has generated an infrared beam of over 2 MW.

Dye laser – Dye laser use dye molecules. By proper tuning, a dye laser is capable of yielding any desired wave length in its range.

Carbon dioxide gas laser – These lasers of about 100 W output emit infrared beam and are helpful in surgery. More powerful CO2 laser are used industrially for the precise cutting of almost any material including metals.

Tiny semiconductor lasers – These lasers process and transmit information. In a CD player a semiconductor laser reads data codes as pits. Semiconductor lasers are ideal for fiber optic transmission lines. Several thousand telephone conversation can be carried by a single fiber cable. Semiconductor laser are used in laser printers.

Excimer laser- Excimer lasers are powered by a chemical reaction involving an excited dimer which is a short lived dimeric or hetro-dimeric molecule formed from two species (atoms), at least one of which is an excited electronic state. They typically produce ultraviolet light and are used in semiconductor photo lithography and in LASIK eye surgery.



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