Many disasters, such as volcanic eruptions, earthquakes, land-slides and clear-air turbulences, radiate infrasound in advance .
By monitoring the infrasound waves , we can forecast these disasters.
How Bad are Infrasonic Waves:
Infrasound is especially dangerous, due to its strong vibrations, or oscillations. They hug the ground, travel for long distances without losing strength, and are unstoppable.
3. Ultrasonic Waves
The longitudinal mechanical waves having frequencies greater than 20000 Hz are called Ultrasonic Waves. But certain creatures like dog, cat, bat, and mosquito can detect these waves. Bat not only detect by also produce ultrasonic waves. Humans cannot hear the sound of inaudible range.
Applications of Ultrasonic Waves:
1. For sending signals.
2. For measuring depth of sea.
3. For cleaning cloths, aeroplanes and machinery parts of clock.
4. For removing lamp-shoot from the chimney of factories.
5. In sterilizing of a liquid.
6. In Ultra-Sonography.
7. Doppler effect: Doppler effect to assess whether structures (usually blood) are moving towards or away from the probe , and its relative velocity .
8. Whales make use of ultrasounds for communication purposes. Individual pods of whales have their own distinctive dialect of calls, similar to songbirds.
Speed of Sound:
Speed of sound is different in different mediums. In a medium, speed of sound basically depends upon elasticity and density of medium.Speed of sound is maximum in solids and minimum in gases.
When sound enters from one medium to another medium, its speed and wavelength changes but frequency remains unchanged. In a medium, the speed of sound is independent of frequency.
Medium Speed of sound (In m/s)
Carbon dioxide
260 Air (0 C) 332 Air (20 C) 343 Steam (100 C) 405
Helium 965
Alcohol 1213 Hydrogen 1269 Mercury 1450 Water (20 C) 1482 Sea Water 1533
Copper 3560
Iron 5130
Glass 5640
Granite 6000 Aluminum 6420
What is Audible Range of Sound?
Sounds with frequency between 20 Hz to 20,000 Hz are called audible sound. The hearing range of human beings is between 20 hertz to 20,000 hertz. Sound with frequency below 20 hertz and above 20,000 hertz is called sound of inaudible range.
Humans cannot hear the sound of inaudible range. Many animals, such as dogs, cats, etc. can hear the sound with frequency above 20,000 hertz
How sound is produced by Humans?
Larynx is composed of two stretched membranes; with some gap between them.
When air passes through the larynx, the membranes or vocal cords vibrate and produce sound. That is why larynx is also known as sound box.
Various Ways Pleasant Sound is Produced:
Some instruments produce sound due to the -vibration of membranes,
-vibration of strings, and -vibration of an air column.
The to and fro or back and forth motion of an object is called vibration. The sitar, veena, violin, guitar and ektara are some stringed instruments.
The tabla, cymbals, ghatam, kartal and manjira are some instruments that work on the vibration of a membrane.
The instruments like the flute and the trumpet produce sound due to the vibration of an air column present in them.
Sonar and its Working Principle:
SONAR means SOund NAvigation and Ranging. Sonar is an instrument that use ultrasonic wave for sound ranging. It measures even short time intervals quite accurately.
Sonar works on the principle of echo. A strong and short (ultrasonic) sound signal is sent towards the bottom of the ocean. Echo of the signal is then detected and depth of ocean is calculated.
What is Echo?
The sound waves received after being reflected from a high tower or mountains is called echo.
To hear echo, the minimum distance between the observer and reflector should be 17m (16.6 m).
Persistence of ear (effect of sound on ear) is 1/10.
Doppler Effect: If there is a relative motion between source of sound and observer.
The apparent frequency of sound heard by the observer is different from the actual frequency of sound emitted by the source. This phenomenon is called Doppler Effect.
When the frequency between the source and observer decreases, the apparent frequency increases and vice-versa.
What is Mach Number?
Mach Number: It is defined as the ratio of speed of sound source to the speed of sound in the same medium under the same condition of temperature and pressure.
If match number >1, body is supersonic.
If match number >5, body is called hypersonic.
If match number < 1 -the body is said to be moving with subsonic speed.
Some reasons commonly asked in Exam:
Children and women produce sound of high frequency and their sound is shriller and of higher pitch. On the other hand, an adult male produces sound of lower frequency and his sound is less shrill and has lower pitch.
A drum produces sound of lower frequency which is less shrill and has lower pitch, while a whistle produces sound of higher frequency which is shriller and is of higher pitch.
A lion produces a sound of lower frequency which is less shrill and has lower pitch, while a bird produces sound of high frequency which is shriller and has higher pitch. However, sound of lion is louder than the sound of a bird.
We know that the speed of light much more than the speed of sound. Due to this, light reaches to us faster than sound. Hence, during lightning we see the streak of light earlier than hearing the sound of thunder.
Due to refraction, sound is heard at longer distance in nights than in day.
Resonance: If the frequency of the imposed periodic force is equal to the natural frequency of a body, the body oscillates with a very large amplitude. This phenomenon is called resonance.
Interference of sound: The modification or redistribution of energy at a point due to superposition of two (or more) sound waves of same frequency is called interference of sound. If two waves meet at a point in same phase, intensity of sound is maximum at that point. Such type of interference is called constructive interference.
Similarly, if the two point meet at a point in opposite phase, intensity of sound at that point is minimum. Such type of interference is called destructive interference.
Diffraction of sound: Wavelength of sound is of the order of 1 m. If an objective of that range appears in the path of sound, sound deviates at the edge of obstacle and propagates forward. This phenomenon is called diffraction of sound.
Shock waves: A body moving with supersonic speed in air leaves behind it a conical region of disturbance which spreads continuously. Such a disturbance is called shock waves. These waves carries a huge energy and may even cracks in window panes or even damage a building.
Bow waves: When a motor boat travels faster than sound, then waves just like shock waves are produced on the surface of water. These waves are called bow waves.
Some Questions:
Which of the following is true? a) Sound waves exhibit interference b) Light waves exhibit interference c) Both the light and sound waves exhibit interference d) Neither sound waves nor light waves exhibit interference.
Effects of Refraction of Light
1. A swimming pool always looks shallower than it really is, because the light coming from the bottom of the pool bends when it comes out at the surface due to refraction of light.
2. A straight stick which is immersed partly in water always looks to be bent at the surface of water, because the light coming from the stick bends when it comes out at the surface due to refraction of light.
3. A coin or stone lying at the bottom of a container filled with water appears to be raised because of refraction of light.
4. A line or a spot of ink on a paper always appears to be raised when viewed through the glass slab due to the refraction of light.
5. Twinkling of stars is due to the refraction of light.
6. Optical illusions such as mirage and looming are also produced due to refraction of light.
Essential Conditions for Total Internal Reflection
There are two conditions which are essential for total internal reflection. These are:
1. The light should travel from a denser medium to a rarer medium.
2. The angle of incidence of light traveling in denser medium should be greater than the critical angle of the medium
Effects of total internal reflection of Light
Twinkling of Stars: The light rays coming from a star reaches our eyes after passing through the atmosphere having different air layers of different optical densities. But the optical densities of different layers of air keep on changing continuously due to change in temperature conditions. Due to which, the light rays coming from a star are refracted to different amount at different moments of time, and the path of refracted rays keep on changing. As a result, sometimes more light is refracted towards our eyes
and the star appears bright to us, whereas sometimes less light is refracted towards our eyes and the star appears dim to us. This gives rise to the twinkling effect of a star.
The Sun is visible to us 2 minutes before actual sunrise and 2 minutes after the actual sunset: The Sun is visible to us 2 minutes before actual sunrise and 2 minutes after the actual sunset due to atmospheric refraction. Actually when the Sun is slightly below the horizon, then the light rays emitted by the Sun are refracted downwards when passing through the optically rarer air layers into the optically dense air layers of atmosphere. Due to which, the Sun appears to be slightly raised above the horizon and is visible 2 minutes before actual sunrise and 2 minutes after the actual sunset.
Mirage : Mirage is an optical illusion which occurs usually in deserts on hot summer days due to atmospheric refraction and total internal reflection of light rays. In mirage, the object such as a tree appears to be inverted as if it is situated on a bank of a pond of water.
Looming: Looming is also an optical illusion which occurs usually in very cold regions. In looming, a distant object such as a ship moving in polar areas appears to be hanging in midair due to atmospheric refraction and the total internal reflection of light rays.
Brilliance of diamond: The brilliance of a diamond is due to the total internal reflection of light. We know that the refractive index of diamond is 2.42, and the critical angle for diamond is 240. The diamond is cut in such a way so that the light which enters the diamond from any face suffers multiple total internal reflections at the various faces before coming out of the diamond. Due to this, the diamond sparkles.
Rainbow (refraction + total internal reflection)