3 & 4- Sound; Principle behind Rainbow UPSC Notes | EduRev

RAS RPSC Prelims Preparation - Notes, Study Material & Tests

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3. SOUND: ECHO, RESONANCE, DOPPLER EFFECT, SONIC BOOM, DOLBY

Sound is produced by vibrating objects. In human beings, the vibration of the vocal cords produces sound. Sound travels through a medium (gas, liquid or solid). It cannot travel in vacuum.

THE DOPPLER EFFECT  

The Doppler Effect (or Doppler shift), named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague, is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.

SONIC BOOM  

A sonic boom is the sound associated with the shock waves created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion. The crack of a supersonic bullet passing overhead is an example of a sonic boom in miniature.

DOLBY NR  

Dolby NR is the name given to a series of noise reduction systems developed by Dolby Laboratories for use in analogy magnetic tape recording. The first was Dolby A, a professional broadband noise reduction for recording studios in 1966, but the best-known is Dolby B (introduced 1968), a sliding band system for the consumer market, which helped make high fidelity practical on cassette tapes, and is common on stereo tape players and recorders to the present day. Of the noise reduction systems, Dolby A and Dolby SR were developed for professional use.

ECHO 

In audio signal processing and acoustics, an echo (plural echoes) is a reflection of sound, arriving at the listener sometime after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single reflection of the sound source. The time delay is the extra distance divided by the speed of sound.

RESONANCE  

Resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others. Frequencies at which the response amplitude is a relative maximum are known as the system's resonant frequencies, or resonance frequencies.

Resonance phenomena occur with all types of vibrations or waves: there is mechanical resonance, acoustic resonance, electromagnetic resonance, nuclear magnetic resonance (NMR), electron spin resonance (ESR) and resonance of quantum wave functions.One familiar example is a playground swing, which acts as a pendulum.

Pushing a person in a swing in time with the natural interval of the swing (its resonant frequency) will make the swing go higher and higher (maximum amplitude), while attempts to push the swing at a faster or slower tempo will result in smaller arcs. This is because the energy the swing absorbs is maximized when the pushes are 'in phase' with the swing's natural oscillations, while some of the swing's energy is actually extracted by the opposing force of the pushes when they are not.

Resonance occurs widely in nature, and is exploited in many man-made devices. It is the mechanism by which virtually all sinusoidal waves and vibrations are generated. Many sounds we hear, such as when hard objects of metal, glass, or wood are struck, are caused by brief resonant vibrations in the object. Light and other short wavelength electromagnetic radiation is produced by resonance on an atomic scale, such as electrons in atoms. Other examples are:

Mechanical and acoustic resonance

   •  the timekeeping mechanisms of modern clocks and watches, e.g. the balance wheel in a mechanical watch and the quartz crystal in a quartz watch 
   • the tidal resonance of the Bay of Fundy 
   • acoustic resonances of musical instruments and human vocal cords 
   • the shattering of a crystal wineglass when exposed to a musical tone of the right pitch (its resonant frequency) 

ELECTRICAL RESONANCE  

    •  electrical resonance of tuned circuits in radios and TVs that allow radio frequencies to be selectively received 

OPTICAL RESONANCE  

   •  creation of coherent light by optical resonance in a laser cavity 

 

4: PRINCIPLES BEHIND RAINBOW, LCD, CAMERA MICROSCOPE, LASER, COMPACT DISC  
 
RAINBOW 

A Rainbow is an optical and meteorological phenomenon that is caused by reflection of light in water droplets in the Earth's atmosphere, resulting in a spectrum of light appearing in the sky. It takes the form of a multicolour arc.

CAMERA MICROSCOPE  

LASER-Laser is the abbreviation of Light Amplification by the Stimulated Emission of Radiation. 
                                                     
It is a device that creates a narrow and low-divergent beam of coherent light, while most other light sources emit incoherent light, which has a phase that varies randomly with time and position. Most lasers emit nearly "monochromatic" light with a narrow wavelength spectrum. 

The principle of a laser is based on three separate features:

a) stimulated emission within an amplifying medium,

b) population inversion of electronics and

c) an optical resonator. 

Spontaneous Emission and Stimulated Emission 

According to the quantum mechanics, an electron within an atom or lattice can have only certain values of energy, or energy levels. There are many energy levels that an electron can occupy, but here we will only consider two. If an electron is in the excited state with the energy E2 it may spontaneously decay to the ground state, with energy E1, releasing the difference in energy between the two states as a photon. This process is called spontaneous emission, producing fluorescent light. The phase and direction of the photon in spontaneous emission are completely random due to Uncertainty Principle.

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