Professor G. S. Mehmi

Echo 1. The repetition of sound heard due to reflection of sound from a large obstacle after the original sound is stopped is called echo. 2. It exists even in open space like mountain's, well, etc. 3. Minimum distance for the echo to be produced is 34 m for short duration sounds and 17 m for other sounds. 4. No such optimum value is for echo. 5. It is not possible to pre-estimate echo in a room or hall. Reverberation 1. The persistance of sound in a room or hall after the original source of sound had stopped emitting sound is called reverberation. 2. It exists only in a room or hall. 3. It may exist in small rooms or halls with large reflecting surfaces. 4. Optimum value of reverberation time for speech is 0.5 second and for music is between 1-1.5 second. 5. Reverberation can be pre-estimated in a room or hall even at the time of designing.

Light waves 1. Light is an agency which produces in us the sensation of sight. 2. Velocity of light in air is approximately 3×10^8 m/s or 186000 miles per second. 3. Light waves are transverse in nature. 4. Light waves can be polarized. 5. Light can not pass through opaque substance. Sound waves 1. The physical cause which enables us to have sensation of hearing is called sound waves. 2. Velocity of sound waves in air is approximately 340 m/s or 1100 feet per second. 3. Sound waves are longitudinal waves. 4. Sound waves can not be polarized. 5. Sound waves can pass through every medium.

Longitudinal  waves 1.  In longitudinal waves, the particles of the medium vibrate back and forth parallel to the direction of propagation of the wave. 2. Longitudinal waves travel in the form of compressions and rarefactions. One compression and one rarefaction constitute one wave. 3. Longitudinal waves are possible in media which possess the properties of elasticity of volume i.e., they are possible in solids, liquids and gases but in gases, this is the only wave motion possible. 4. Longitudinal waves can not be polarised. 5. Pressure of medium changes when longitudinal displacement passes through it, but there is no transverse displacement in the medium. 6. Longitudinal waves cannot be represented by sine curve directly. 7. Examples: Sound waves in air, gases and waves in springs are longitudinal waves.                      Transverse waves 1. In transverse waves, the particles of the medium vibrate in a direction...

LASER means "LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION." It is a device which is used for producing highly intense, strongly monochromatic, highly coherent and collimated beam of light. Characteristics of laser : (1) Monochromatic Light: A light of single wavelength or frequency is called monochromatic light. Laser light is highly monochromatic light i.e. all the photons have same energy above the ground energy state E1 and same frequency i.e.f=(E2 - E1)/h where E2 is the energy of excited state and h is Planck's constant. (2) Coherent light:  A light in which all the photons or waves are in same phase with one another is called coherent light. Laser light is highly coherent in nature. * Incoherent light: The individual photons are not in phase with one another is called  incoherent light. (3) Intense light: A coherent light in which all the photons are in phase is called an intense light. The amplitude of coherent wave is n × a and the resultant int...