Introduction
Have you ever thought, why a camel can run easily in desert? Or an army tank easily runs upon the continuous chain etc. In all of these cases, an object is exerting force on one another object when coming in contact with each other, making the thing to move on. The force applied by an object on another, depends on the area of contact with each other. Thus, the pressure is defined as the forces exerted by an object per unit area of the surface.
If we stand on the loose sand, the force equivalent to the weight of the body acts perpendicular to the surface, equal to the area occupied by our feet. On the other hand, if we lie down on the sand, the force will act on the surface equal to the area of the body in contact with the sand. We therefore see that the effect of same force of equal magnitude is different over different area. Hence, we can say that the pressure acting on the surface depends on the area of contact with the surface. If we press an iron sheet with the help of our finger, our finger does not dip into the sheet as the force of finger is falling on a large area of the sheet and the force per unit area of the sheet is small. The pressure is given by:
\[Pressure=\frac{Force}{Area}\]
Or, \[P=\frac{F}{A}\]
The Sl unit of pressure is Newton per meter, which is also called Pascal (Pa).
1 Pascal = 1 Newton per square meter
Or, 1 pa = 1 N/m2
Pascal is a small unit of pressure. The bigger unit of pressure is kilopascal or cap. One kilopascal is equal to the one thousand Pascal.
i.e. 1 kPa = 1000 pa
In real life, there are many such situations where the pressure is the most important parameter. If you are peeling a potato, pressure is the key factor. If the knife is sharp, the area of contact is small and you can peel with less force on the blade. But if the knife is not sharp, you have to apply more pressure to peel it off. Similarly, if a nurse is giving injection to the patient and the needle is sharp, she has to apply less pressure on the syringe. In such case, the patient will feel less pain, as the sharp needle will have lesser area of contacts and less force is required to push the needle through the skin. It is normally seen that the foundation of the building is kept very wide. This is done in order to provide strength to the building.
Thus, we can say that same force acting on the smaller area exerts larger pressure than the force acting on the large area.
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Types of Waves
There are normally two types of waves:
(i) Longitudinal wave
(ii) Transverse wave
The nature of the wave depends on the elastic and inertial properties of the medium.
Longitudinal Waves
It is the wave, in which the particles of the medium vibrates in the direction of propagation of the wave. In this process the particles do not move with the wave, they simply oscillate back and froth about their individual equilibrium position. In the figure given below, the wave is seen as the motion of the compressed region, which moves from left to right. The region in which the particles have high density is called compression and the region in which the particles have low density is called the rarefactions. The distance between any two compressions and rarefactions is called its wavelength.
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Transverse Waves
The wave, in which the particles of the medium vibrates in the direction perpendicular to the direction of their propagation, is called the transverse waves. The particles do not move along the wave, they simply oscillate up and down about their individual equilibrium position, as the wave passes through the medium.
There are different types of waves produced by different sources. The wave produced by the home theater is different from that produced by the light of bulb. What is the nature of sound wave produced by the home theater?
(a) Transverse
(b) Plane
(c) Longitudinal
(d) All of these
(e) None of these
Answer: (c)
Explanation
The nature of the sound wave produced by the home theater is longitudinal.
The wave, which travels in a straight line and cannot bend round the corner is called transverse wave. Which one of the following is an example of transverse wave?
(a) Sound wave
(b) Light
(c) Music
(d) Electromagnetic wave
(e) None of these
Answer: (b)
Explanation
We know that light wave is transverse in nature.
Reflection of Sound
When we shout in a well or in an empty room, we hearoursound after sometime. This is known as the reflection of sound. When the sound strikes any solid object, it reflects back. If the surface is a smooth one, the reflection is more, and if the surface is rough, the reflection is less. On the rough surface, some part of the sound is absorbed by the surface. There are two phenomena of reflection:
(i) Echo
(ii) Reverberation
Echo
If a sound produced is reflected back after some time on striking a solid surface, it is known as echo. The condition for echo to occur is that, the object should be at a minimum distance of 15 m. The distance for an object of 2 sec echo return would be 343 meters, as the speed of the sound is 343 m/sec. In most cases of human hearing, the echo is about one-half of a sec or about half the distance. The sound grows fainter with the distance.
Reverberation
The repeated reflection of sound is called reverberation. The reflection time in case of reverberation is approximately 0.1 sec, which is much lesser than the echo. In case of reverberation the sound cannot be recognized. It is also defined as the collection of reflected sounds from the surface in any closed space, like auditorium. In an auditorium, it is desirable that the reverberation should be of such an extent that it helps to overcome the inverse square law drop-off of sound intensity. If the reverberation is excessive, it makes the sound runs together with loss of articulation, that is, the sound become unrecognizable and garbled.
Reverberation is resulted from reflection of any sound wave from obstacles and its return to the point of listening. That is why we perceive a direct source of an acoustic sound and its multiple reflections from the closest objects.
The signal with highest intensity comes first and is followed by the sound with lower intensity. In real situation, the sound impulses are lengthier than the time of arrival of first reflections. Hence, reverberation is superimposed on a source sound.
There are various phenomenon of reflection of sound. The reverberation is one of them. In case of reverberation the sound is not distinct and cannot be understood properly. This is the reason, while constructing the auditorium, the reflection time is taken care off . What is the reflection time in case of reverberation?
(a) Less than 0.1 sec
(b) 0.1 sec
(c) More than 0.1 sec
(d) 0.5 sec
(e) None of these
Answer: (a)
Explanation
The reflection time more... 
Components of Sound Wave
The sound wave has got the following components:
Amplitude
The maximum displacement of the waves from the mean axis is known as amplitude. The amplitude of the waves tells us about the intensity of the sound.
We can increase or decrease the amplitude of vibration according to our wish.
Time Period
Time taken by the wave to complete on vibration is called time period. It is denoted by 'T. It can also be defined as the time taken by the sound wave to complete one oscillation.
Frequency of Vibration
It is the number of vibration made by the wave in one second. It is measured in Hertz. If an object makes one vibration in one second then its frequency is one Hertz. To find the frequency of a wave, we always divide the total number of frequency in a given time, by the total time taken.
Wavelength
The distance between any two consecutive crest or trough is called wavelength.
Relation between Frequency and Time Period
The frequency of vibration is given by the number of vibration in one second, that is, the frequency is equal to the reciprocal of the time period.
\[Time\text{ }period=\frac{1}{frequency}\]
\[T=\frac{1}{f}\]
Relation between Wavelength and Frequency
If V is the velocity of the wave and^, is the wavelength of the wave, the velocity of the wave is given by:
Velocity = Wavelength x Frequency
V=Xxf
The frequency of the sound traveling with a velocity of 680 m/sec and a wavelength of 2 m is:
(a) 340 Hz
(b) 240 Hz
(c) 520 Hz
(d) 680 Hz
(e) None of these
Answer: (a)
Explanation
The frequency of the sound can be calculated by using the relation,\[v=\lambda \times f\].
On calculation it is found to be 340 Hz.
Therefore, option (a) is correct and rest of the options is incorrect.
The maximum upward elevation of the wave about its mean position is called:
(a) Trough
(b) Crest
(c) Wavelength
(d) Frequency
(e) None of these
Answer: (b)
Explanation
The maximum upward displacement of the wave about its mean position is called crest. 
Characteristics of Sound
A sound normally has three characteristics, which helps us to recognize the sound. These three characteristics are:
(i) Loudness
(ii) Pitch
(iii) Quality
Two different sounds differs in at least one of the three given characteristics.
Loudness
Sound is produced by the vibration of objects. If more energy is supplied to the object it will vibrate with larger amplitude, and the loudness of the sound will be more. Thus, the loudness of the sound depends on the amplitude of the vibration. If the amplitude of the sound is doubled, the loudness of the sound will be four times, and if the amplitude of the vibration is halved, the loudness of the sound will be one-fourth of the original sound. The loudness of the sound is measured in decibels. It is denoted by dB. The loudness of the normal conversation is 60 dB and that of the normal breathing is 60 dB. If the loudness of the sound is more than 80 dB then it is called sound pollution or painful sound. The loudness of the jet aero plane is 130 dB.
Pitch
The pitch of the sound helps us to differentiate between the sounds of same loudness. For example, the sound of men and women of same loudness can easily be differentiated. The pitch of the sound depends on the frequency of the sound. A sound having high frequency is called shrill. The sound of women is shriller than men.
Quality
Quality is that characteristic of sound, which helps us to differentiate between the sounds of different instruments. The sound produced by sitar, tabia, flute, etc. are different from each other, even if their pitch and frequency are same. This difference in the quality of different sound is because of the shape of the waves, which is different in different instruments.
The sound produced by the different instruments are different. There are different characteristic of sound, which help us to different between the different sounds. The characteristic of sound which helps us to differentiate between the sounds of different instruments is called:
(a) Pitch
(b) Loudness
(c) Quality
(d) Tone
(e) None of these
Answer: (c)
Explanation
We can easily differentiate between the sounds of different instrument because of its quality.
The sound is measured in decibels. It is also called the unit of intensity of sound. The loudness of normal conversation of human is:
(a) 10 dB
(b) 30 dB
(c) 60 dB
(d) 80 dB
(e) None of these
Answer: (c)
Explanation
The loudness more... 
Human Ears
Ear is a sense organ that helps in hearing the sound. The human ear is divided into three parts:
(i) Outer ear
(ii) Middle ear
(iii) Inner ear
The outer ear consists of pinna. It collects the sound from the outer surroundings and transfers it to the middle ear, through the ear canal. At the end of the ear canal there is a thin elastic membrane, called eardrum. The middle ear consists of three bones/hammer/ anvil and stirrup. The inner ear contains a coiled tube called cochlea. Cochlea is filled with a liquid that contains nerve cell, which is sensitive to the sound. They are connected with the auditory nerves that transmits the message to the brain.
The sound wave coming from the outer surrounding is collected by the outer ear, called pinna. These sound waves are then pass on to the middle ear through the ear canal, which then fall on the eardrum. The eardrum starts vibrating rapidly, in turn causing the three bones to vibrates. The three bones amplify the sound from outer ear to several times and then pass it on to the inner ear. This cause the liquid in the cochlea to vibrate, and sets up the electrical impulse in the nerve cell. The electrical impulse is then carried by auditory nerves to the brain. The brain interprets these impulses as sound, and we get the sensation of hearing.
The sound from the surrounding entering into our ear are collected by pinna. The sound which enters into our ear is not of high amplitude. They are amplified inside our ear. The part of the ear which does this work is?
(a) Ear Drum
(b) Hammer, Anvil and Stirrup
(c) Ear Canal
(d) Cochlea
(e) None of these
Answer: (b)
Explanation
The middle part of the ear amplifies the sound and it consists of three bones, hammer, anvil, and stirrup.
The sound from our ear is carried to our brain, where it is interpreted and we are able to hear and identify the sounds. Which part of the ear carries the sound message to the brain?
(a) Optic Nerve
(b) Sensory Nerve
(c) Motor Nerve
(d) Auditory Nerve
(e) None of these
Answer: (d)
Explanation
The part of ear that carries the message from the ear to the brain is auditory nerve. 
Propagation of Sound
Sound is propagated in the medium due to vibration produced in the medium. When an object starts vibrating in the medium, the air molecules in close contact with object also starts vibrating back and froth with the same frequency. These vibrating air molecules pass on their motion to the next layer of the air due to which they also starts vibrating. This process goes on. In this way the vibration reaches our ears and we hear the sound.
In human, sound is produced by the voice box present in their throat, which is also known as the larynx. The larynx contains two ligaments, known as vocal chord. The sound is produced by the vibration of the larynx. When we speak or sing, the frequency of the sound changes continuously. This happens due to the action of the muscles attached with the vocal chord. If the muscles attached to the vocal chord contracts and stretched, the vocal chord becomes tight and thin and the sound produced is of high frequency. On the other hand, if the muscles relax, the vocal chord becomes loose and the sound produced is of low frequency. The vocal chord of man is 20 mm and that of women is 15 mm. Thus, the voice of woman is softer than man. Hence, due to the difference in the length of the vocal chord, the voice of man, woman, and children are of different frequencies.
Sound Needs Medium for Propagation
The matter through which sound travel is known as medium. It can be solid, liquid orgas. The sound needs medium to travel. It cannot travel in vacuum, as vacuum do not have any molecules to vibrate. Thus, for propagation of sound material medium is necessary. If we go to the outer space, we cannot talk as we talk on the Earth. This is because there is no medium in the outer space. That is why the astronauts use wireless, i.e. radio waves to talk with each other in the outer space. The speed of the sound is different in the different medium. This is because, the density of different medium is different. It is fastest in solid and slowest in gas;The speed of sound is 340 m/s in air, 1500 m/sec in water and 5000 m/sec in air. But if we compare the speed of light with that of sound, we find that light travels faster than sound. For example, during rainy days, with thunder storm and lightning, we see the light first and hear the sound of thunder little later.
Sound travels with different speed in different medium. The speed of sound is different due to the fact that, different medium has different density. In which one of the following medium sounds will travel fastest? more... 
Introduction
CRASH! BANG! We often hearthat sound. These are actually made by vibrating air. The sounds made by musical instruments are also of same nature. The difference between NOISE and MUSIC is that, mDsical sounds are organized into patterns that have pitch and rhythm. Noise is just random, disorganized sounds. Sounds are made and travel in the same way, whether they are musical or noise.
A pleasing sound is called tone. It is produced by air vibration, occurs certain number of times per second. These vibrations are called waves. The sound wave is controlled, so that the loudness, quality of the tone, and duration of the plays is controlled. Most musical instruments have string, or some other devices that create sound waves, when vibrated. Different sounds are produced because of harmonic. The higher and lower sounds are mixed in to produce a tone of different pitch. The number of times that a sound wave vibrates per second, is called frequency. The frequency of the sound is measured in cycles and called hertz. High notes have a higher frequency and the lower notes have lower frequency. The image given below shows the 'look' of sound waves:
From above, we observe that even if the pitch and volume changes, the shape of the sound wave remains the same.
Some instruments produce sound when blown into, such as, the brass instruments. The vibration of the player's lips make the column of airto vibrate; whereas the woodwinds have reeds, which vibrate when the player blows on it, making the column of airto vibrate. The sound is produced in the flute when the player blows across the mouthpiece, causing vibration of the column. The image below shows the 'look' of the sound waves of different instruments:
Let us See the Following Activities
(1) Wrap some stiff tissue paper around a comb. Hum on the paper and feel the vibration as it shivers.
(2) Beat an empty metal tin with a spoon and feel its vibration as it makes a sound.
(3) Fill an empty glass jar with water upto the brim. Take a pencil and gently tap it near the rim. Also fill other jar with different amounts of water and hear the sound in both case. You will find that the sound produced in both the case is different. Thus, we can say that more the water in the jar, lower is the pitch, because the vibrations are slower. On the other hand, less the water in the jar, higher is the pitch, because the vibration is more.
We can, therefore, define sound as the form of energy that produces the sensation of hearing.
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Power of a Lens
A lens either converges or diverges the ray of light. The ability of lens to convergeor diverge the ray of light is called the power of lens. The convex lens convergesthe ray of light and the concave lens diverges the ray of light. The power of thelens depends on the focal length of the lens. It is also defined as the reciprocalof the focal length. It is given by,
\[Power=\frac{1}{Focal\,length},or\,\,\,P=\frac{1}{F}\]
Here, P is the power of the lens and F is the focal length of the lens.
The unit of the power of the lens is Diopter. It denoted by D. One diopter is thepower of the lens whose focal length is one meter. If the focal length of thelens is positive, it is a convex lens, its power is positive. On the other hand, ifthe focal length of the lens is negative, it is a concave lens, the power isnegative.
If the number of lens is placed in close contact with each other, the power ofthe combination of the lenses is equal to the algebraic sum of the power of theindividual lens. Thus, if P1, P2, P3…….. , Pn are the powers of n-lens, the resultantpower is given by:
\[P={{P}_{1}}+{{P}_{2}}+{{P}_{3}}+---{{P}_{n}}\]
Two lens of power 4.5 D and - 3.5 D are combined together. Find thepower and the focal length of the combined lens.
(a) 2 D and 1m
(b) 3D and 2m
(c) ID and 1m
(d) 2 D and 2 m
(e) None of these
Answer: (c)
Explanation
The combined power of the number of lens is equal to the sum of theindividual powers of the lens.
A convex lens of focal length 40 cm and a concave lens of focal length200 cm are placed in contact with each other. Find the power of thecombined lens.
(a) 2 D
(b) 3 D
(c) 1.5 D
(d) 2.5 D
(e) None of these
Answer: (a)
Explanation
On calculation we find that the combined power of the lens is 2 D.
Dispersion of Light
We know that the white light consists of seven colours. When it is passedthrough a prism, it splits into its constituent colours known as VIBGYOR. Thisband of seven colours is called spectrum. The seven colours of the spectrumare violet, indigo, blue, green, yellow, orange, and red. The red colour is at thetop of the spectrum, while the violet colour is at the bottom of the spectrum.The dispersion of white light as it passes through the prism is due to thedifference in the angle of refraction of light, of different colour. The sequenceof the colours in the spectrum are in the decreasing order of their wavelengthfrom top to bottom. One of the various effects more... 
Refractions of Light
We know that the light travels with different speed, in different mediums. Italways travels in straight line. When it travels from one medium to another, itchanges its direction. The change in the direction of light, as it passes fromone medium to another, is known as refraction of light. If it goes from rarermedium to denser medium, it bend towards the normal. If it goes from densermedium to rarer medium, it bends away form the normal.
The speed of light in air is 3 x 108 m/sec and that in the glass is 2 x108 m/sec.
The speed of light in water is 2.25 x108 m/sec.
Thus, we can define the refractive index of the medium as the ratio of speedof light in the medium, to the speed of light in the air. It is also called theSnell's law. It is given by,
\[\mu =\frac{\sin i}{\sin r}\]
Various Effects of Refractions of Light
(a) A stick immersed in water appears to be bent at the surface of thewater.
(b) The bottom of the swimming pool appears to be less deep, thanactually it is.
(c) Stars appear twinkling in the sky during the night.
(d) Appearance of the pool of water in the desert during summer, whenobserved from the long distance.
Lens
It is the transparent material having two refracting surfaces. It is of two type,concave lens and convex lens. The lens, which is thicker at the centre andthinner at the edge is called convex lens. It is also called converging lens. Onthe other hand, the lens which is thinner at the centre and thicker at the edgeis called concave lens. It is also called diverging lens.
Dotted lines are where light appears to be corning from when seen from right-hand side of the lens.
Laws of Refraction of Light
There are two laws of refraction of light:
(a) The incident ray, the refracted ray, and the normal, all lies on thesame plane.
(b) The ratio of angle of incidence and the angle of refraction is aconstant.
Formation of Imageby Convex Lens
