Wednesday, May 16, 2012




                Thermodynamics is the branch of science which deals with energy (heat) transfer and its effect on state or condition of a system. Essentially thermodynamics pertains to the study of:

1.   Interaction of system & surrounding:-

      It relates the changes which the system undergoes to the influences to which it is put.

2.   Energy & its transformation:-

                                Energy inter-conversion in the form of heat & work. James Joule had proved with his well known experiment that mechanical work can be converted into heat energy. The credit for using heat by converting into work goes to James Watt who produced the first "Steam Engine" & paved the way for industrial revolution.

3.   Relationship between heat, work & physical properties.

                Such as pressure, volume & temperature of the working substance (water for steam engine, petrol for bike etc) employed to obtain energy conversion.
                Thermodynamics has been excellently defined as the science of three "Es" namely Energy, Entropy & Equilibrium. The principles & concepts of thermodynamics are important tools in the innovation, design, development & improvement of engineering processes, equipment & devices which deals with effective utilization of energy.
The applications of engineering thermodynamics in the field of energy technology are as follows:-
1.    Power producing devices for e.g. internal combustion engine (ICG) & gas turbine, steam & nuclear power plant.
2.    Power consuming devices for e.g. fan, blower, compressor, refrigeration & air conditioning plants.
3.   Chemical process plants & direct energy conversion devices. A large number of processes in various fields such as agriculture, textiles, dairy, drugs & pharmaceutical industries are also governed by thermodynamics principle.

Macroscopic & Microscopic approach.

or Classical & Statistical  Thermodynamics.

                There are two approaches for investigating the behavior of a system.

Macroscopic approach.

It is also known as classical thermodynamics which is concerned with gross or overall behavior of matter.  In classical thermodynamics ,the analysis of thermodynamics  systems is explained with the measurable property like pressure ,volume, temperature  etc. It doesn’t explain the structure of matter.(i.e No attention is focused on the behavior of individual particles constituting the matter.)The volume considered is very large compared to molecular dimension & the system is regarded as bulk (whole/single).The study is made of overall effect of several molecules. The behavior & activities of the molecules are averaged. Only a few variables are needed to describe the state or condition of matter. The properties like pressure, temperature etc. needed to describe the system can be easily measured & felt by our senses. The particles of system required simple mathematical formula for analyzing the system for e.g. Piston cylinder assembly of an IC engine, volume occupied by the gas for each position of piston etc. It is based on the concept of continuum. The macroscopic form of energy are those a system possess as a whole with respect to some outside reference frame, such as potential and kinetic energies.

Microscopic point of view

This is also known as statistical thermodynamics and is directly concerned with structure of matter. It focuses on statistical behavior of mass consisting numerous individual molecules and correlates macroscopic properties of the matter with molecular configuration and intermolecular forces. It explains the structure of matter. Few coordinates are not sufficient to describe this system. Concept  of continuum is not valid for this viewpoint. The microscopic form of energy are those related to the molecular structure of a system and the degree of molecular activity, and they are independent of outside reference frames. The sum of all the microscopic forms of energy is called the internal energy of a system.


Thermodynamics system:-

It is defines as a definite area or a space where some thermodynamic process takes place. It is a region where thermodynamic process are studied. A thermodynamic system has it’s boundary and anything  outside the boundaries is called it’s surrounding. These boundaries may be fixed like that of a tank enclosing a certain mass of compressed gas, or movable like boundary of a certain volume of liquid in a pipe line.



Thermodynamic system represents a prescribed & fixed quantity of matter under consideration to analyze a problem; to study the changes in its properties due to exchange of energy in the form of heat & work. The system may be quantity of steam, a mixture of vapor & gas or a piston cylinder assembly of an IC engine & its contents.
                For the description of thermodynamic system some of the following quantities need to be specified.
1.    Quantity as well as composition of matter.
2.    Measurable properties such as pressure, temperature & volume of the system
3.    Energy of the system
                The combination of matter & space, external to the system that may be influenced by the changes in the system is called surrounding or environment. The thermodynamic system & surrounding are separated by an envelope called boundary of the system. The boundary represents the limit of the system and may be either real or imaginary and may change shape, volume, position or orientation relative to observer for e.g. an elastic balloon which is initially spherical in shape may change into cylindrical shape or some other geometrical shape or may get squeezed to reduce volume during a certain period. As such, the boundary of the gas in the balloon couldnot retains the same size and shape. Further, the boundary may be diathermal or adiabatic depending upon whether it allows or not exchange of energy in the form of heat. The walls of the boundary which does not let heat transfer to take place across them are named as adiabatic. In contrast the walls that do allow heat interaction across them are called diathermic. The diathermic system can be classified into.

A.   Closed system

A close system can change energy in the form of heat & work with its environment but there is no mass transfer across the system boundary. The mass within the system remains the same and constant, though its volume can change against flexible boundary further the physical nature & chemical composition of the mass may change. Thus, a liquid may evaporate, a gas may condense or a chemical reaction may occur between two or more components of the system for e.g.
Cylinder fitted with movable piston.
1.                        Motor car battery.
2.                        Pressure cooker
3.                        Refrigerator
4.                        Ice-cream greaser
5.                        Bomb Calorimeter etc.

B.   Open system:

A open system has mass exchanged with the surrounding along with transfer of energy in the form of heat and work. The mass within the system doesn't necessarily remain constant. It may change depending upon mass inflow & mass outflow for e.g.
1.         Water wheel.
2.         Motorcar engine:-
         The engine initially draws charge (mixture of air and petrol) and finally exhausts the spent up gases to the surrounding atmosphere. The mass flows across the engine boundary.
3.         Steam generator (Boiler):-
                A boiler is a device which converts water from liquid vapor freeze. Here consist of system change, water froze into and steam froze out of the system.
Most of the engineering devices are open system.

C.   Adiabatic system:

                There exists wall or boundary which doesn't allow heat transfer to take place across them, no matter how not one member is compared to other. Such boundaries are named as adiabatic. An adiabatic system is thermally insulated from its environments. It is enclosed by adiabatic walls and can exchange energy in the form of work only for e.g. a pipe carrying heat enclosed by thermal insulation.

D.  Isolated system:

An isolated system is a fixed mass and energy; it exchanges neither mass nor energy with another system or with surrounding. An isolated system has no interaction with the surrounding, it neither influences the surrounding, nor it is influenced by it. When a system and its surrounding are taken together they constitute an isolated system. The universe can be considered as an isolated system & so is the fluid enclosed in the perfectly insulated closed vessel (thermos/flask)

E.   Homogeneous & heterogeneous:

A system consisting of single phase is called homogeneous system for e.g.
1.    Ice, water, dry saturated steam.
2.    A mixture of air and water vapor
3.    Mixture of ammonia in water
A system whose mass content is non uniform through i.e. it consists of more than one phase is called heterogeneous system for e.g.
1.    A mixture of ice and water
2.    A mixture of water & gasoline
3.    A mixture of water & mercury

Thermodynamic property

                Thermodynamic property refer to the characteristic which can be used to describe the condition or state of the system for e.g. temperature, pressure, chemical composition, color, volume, energy etc. The salient aspects of thermodynamic properties are:-
a)        It is a measurable characteristic describing system & helps to distinguish one system to another
b)        It has a definite unit value when the system is in a particular state
c)         It is dependent only on the state of the system, it does not depend on path or route. The system follows to attain the particular state.
d)        It's differential is exact
If 'P' is the thermodynamic property with dP representing its differential change, then the integral between initial state 1 to final state 2 of system will have only one value given by 'O2'
Since, the thermodynamic property is the state of a system; it is referred to as point a function or state function. There are two kinds of thermodynamic property namely Intensive and extensive.

  a)Intensive Property:
It is independent of the extent or the mass of the system. Its value remains same whether consider whole the system or part on it for e.g. pressure, temperature, density, viscosity, composition, thermal conductivity, electrical potential etc.
b) Extensive property:
It depends on the mass or extent of the system. Its value depend on how big portion of the system is being considered for e.g. energy, enthalpy (heat contained), entropy (disorder), volume etc.

Specific property:-

                An extensive property expressed per unit mass of the system is known as specific property for e.g. specific volume, specific energy, specific entropy, specific enthalpy etc.


                Equilibrium is the concept associated with the concept of tendency for spontaneous change in the value of any macroscopic property of the system when it is isolated from its surrounding.

1.       Mechanical equilibrium (Equality of pressure)

                Condition or state in which there is no unbalance force within the system and nor its boundaries. Mechanical equilibrium implies uniformity of pressure i.e. there is one value of pressure for the entire system; diffusion takes place to wipe out the unbalance and attain the state of mechanical equilibrium.

2.       Chemical Equilibrium:-

                A system in chemical equilibrium may undergo a spontaneous change of internal structure due to chemical reaction or diffusion (transfer of matter) for e.g. there will be spontaneous change in the properties of the mixture of oxygen & gasoline once it is ignited. Chemical equilibrium represents that condition a state of the system when all chemical reaction in it have ceased & there is no mass diffusion.

3.       Thermal Equilibrium

A system is said to be in thermal equilibrium, when is no temperature difference between the parts of the system or between the system and surroundings.

4.       Thermodynamic Equilibrium

                A system which is simultaneously in a state of mechanical equilibrium, chemical equilibrium & thermal equilibrium is said to be thermodynamic equilibrium.

State, path, process cycle.

Consider a system constituting by a gas enclosed in a piston cylinder assembly of reciprocating machine. Corresponding to the position of the piston at any instant, the condition of the system will be prescribed by pressure, volume & temperature of the gas. When all such properties have a definite value, the system is said to be exist at definite state. State is thus condition of the system identified by thermodynamic property.
When a piston move outwards the properties of the system change (pressure decreases, volume increases). Any such operation in which properties of the system change is called change of state. The locus of series of states through which a system passes is going from initial state to final state constitutes the path.
A complete specification of the path is referred to as process.
                               When a system in a given state undergoes  series  of processes such that the final state is identical with initial state, a cyclic process or a cycle is said to be be executed. The cycle 1-A-2-B-1 consist of 1-A-2 & 2-B-1(process), cycle 1-C-2-B-1 consist of 1-C-2 & 2-b-1 (process).



Quasi-state or Quasi Equilibrium process

  Some unbalanced potential must exist either within the system or between a system & its surrounding to promote the change of state during a thermodynamic process. Consider a system of the gas in cylinder fitted with piston  upon which are placed many small pieces of weights.
                The upward force exerted by the gas just balances the weight on the piston. The system is initially in equilibrium in equilibrium state indentified by pressure P1, Volume V1 & Temperature T1. When these weights are removed slowly one at a time, the unbalanced potential is infinitesimally small. The piston will slowly move upward & at particular instant of piston travel, the system would be almost closed to the state of equilibrium.  The departure of system from thermodynamic equilibrium state will be infinitesimal small. Every state passed by the system will be in equilibrium state.  The locus of series of such equilibrium state is called Quasi-static or Quasi Equilibrium process.
Thus, when the process is carried out in such a way that at every instant, the system deviation from the thermodynamic equilibrium is infinitesimal, then the process is known as quasi-static or quasi equilibrium process. It is a very slow process and each state in the process may be considered as an equilibrium state. It is also known as reversible process.


 Mechanical Oscillation  


The motion which repeats after a regular interval of time is called periodic motion. In such a motion the particle always come to rest is called mean or equilibrium position. If the particle is displaced from mean position there exist certain kind of force which tries the particle back to its mean position, such a force is called restoring force. Restoring force is the function of displacement. Periodic motion can be expressed in terms of trigonometric function. So, it is called harmonic motion.


                Simple harmonic motion is the special kind of  oscillatory motion  in which the body moves  again and again over the same path about the fixed point (generally called equilibrium position) in such a way that it is acted upon by a restoring force ( or torque) propotional to it’s displacement ( or angular displacement) and directed towards the mean position.
SHM is a special type of periodic motion in which particle oscillates in straight line in such a way that acceleration is directly proportional to the displacement from mean position & it is directed towards mean position.



1.       Amplitude:-
                The maximum displacement of the particle from mean position is called amplitude of the oscillation.
As the particle displacement is, x=Asin(ωt+δ) and sin(ωt+δ) varies from -1 to +1
∴ Maximum displacement =±A
A is called amplitude of oscillation.
2.       Time period:-
                The time required to complete one oscillation is called time period. It is denoted by T.
We have, displacement of particle in SHM as, , x=Asin(ωt+δ)…(i)
The position of particle will be same after time T  i.e. x=Asin[ω(T+t)+δ]….(ii)
∴ sin(ωt+δ) repeats its value every 2π or even integral of π.
∴ ω(T+t)+δ = ωt+δ+2π
or,  ωT=2π
This is the time period of S.H.M.
3.       Frequency:-
The number of oscillation in one second is called frequency. It is denoted by 'f' and given as,
4.       Angular frequency (ω):-
5.       Phase & phase constant:-
Phase is the status of the particle which executes S.H.M.
We have,            x=Asin(ωt+δ) and v=Aωcos(ωt+δ)
(ωt+δ) is responsible for the status of particle therefore it is phase of the particle. It is denoted by ϕ.
Phase (ϕ)=ωt+δ
The phase increases with time.
δ is called phase constant which depends upon the choice of instant time (t=0). If we chose the instant time as origin then, ωt+δ=0   ∴ δ=0
                               This means phase constant will be zero.


                Consider a spring of length 'l' which is supported by a fixed rigid support at one end and another end is free. Let 'm' be the mass attached to free end. Due to mass 'm' let 'l' be the elongation produced then, according to Hook's law, the force applied is directly proportional to the extension produced,
i.e. F1∝ l
or, F1=-kl…(i)
-ve sign indicates that the force is restoring.

Now, further pull the mass through a distance 'x' then, the force will be,
∴ the net force applied so that the spring mass system sets into oscillation is,
or, F=-k(l+x)-(-kl)
or, F=-kx….(iii)
According to Newton;s law,
from (iii) & (iv)
which is equation of S.H.M i.e. the spring mass system executes S.H.M.
This is time period of spring mass system.



Angular harmonic motion is the periodic motion in which angular acceleration is directly proportional to angular displacement & it is always acting towards mean position.
                If θ be the displacement & α be the angular acceleration, then,
                α ∝ θ
or, α =-ω²θ        


                Limitation of simple pendulum
a.       The point mass heavy bob is not possible.
b.       The mass less inextensible spring is not possible.
c.       In simple pendulum the centre of oscillation & centre of gravity lies at the same point which is practically not possible
d.       Since the string has certain mass, it has certain moment of inertia which is not considered in it.
e.       It is an ideal pendulum.

Compound pendulum is a rigid body of any shape capable of oscillating in horizontal axis in vertical plane not passing through centre of gravity.

Let 'mg' be the weight acting vertically downward through c.g. Let 'l' be the length of compound pendulum i.e. distance from centre of suspension to centre of gravity(G). Now, displace the pendulum through angle 'θ' & G' be the new position of c.g. , then the force at G'and its reaction at c.s constitutes couple.
Then, torque is given as,
                Torque (τ)=Force × perpendicular distance from axis of rotation
                or τ = mg × lsinθ
If the displacement is small then,
sinθ ≃ θ
∴ τ=mglθ
which provides restoring force i.e. which tries the pendulum back to its mean position.
                ∴ Restoring torque (τ)= -mglθ………(i)
                If I be the moment of inertia and α be the angular acceleration, then,
                Torque (τ) = Iα ….(ii)
From (i) & (ii)
Iα = -mglθ
i.e. angular acceleration is directly proportional to angular displacement or it executes angular harmonic motion.
                comparing with α =-ω²θ , we get,


                If I0 be the moment of inertia through then, I0=mK²
                where, K is radius of gyration. It is perpendicular distance from the centre of suspension to centre of gravity.
                Then moment of inertia through C.S., I=I0+mass × distance²
or, I=mK²+ml²
If L is the length of simple pendulum then, the time period of compound pendulum & simple pendulum will be same. This length is called length of equivalent simple pendulum.
Since, k² is always +ve   is always greater than length of compound pendulum l.
This implies that the centre of oscillation always lies beyond the centre of gravity.

*Interchangeability of centre of suspension & centre of oscillation.

We have, time period of compound pendulum,

Now, inverting the centre of suspension & centre of oscillation we get,                                                    
Then, time period of compound pendulum will be,

Time period is same when it is inverted i.e. the centre of oscillation & centre of suspension can be interchanged.
* Maximum & minimum time period of compound pendulum.
We have, time period of compound pendulum
Tmax=∝ at l→0                                      
Now squaring eqn. (i) we get,
Differentiating eqn. (ii) w.r.t. l,
Tmin=0 at K=±l

Time period of compound pendulum is minimum when radius of gyration 'K' is equal to the length of compound pendulum.
* Energy conservation is S.H.M.
E= KE + PE (constant)
The equation of motion for particle executing S.H.M. is written as F=-kx….(i)
Where, F=restoring force, K=positive constant & x= displacement of particle.
The amount of work done to displace a particle through distance dx will be=> dW=F× dx….(ii)
from (i) &(ii) , dW=-kx × dx
The total work done to displace x distance is,
∫ dW = ∫-Kxdx
This amount of work done is changed into potential energy.
If 'm' be the mass & 'v' be the velocity of particle executing S.H.M. then,
but,        x = A.sin(ωt+δ)
 , which is independent of time i.e. total energy remains conserved.


Q1. 2 Kg. mass hang from a spring. A 300g body hang below the mass stretches the spring 2 cm further. If the 300g body is removed and the mass set onto oscillation. Find the period of motion.
or, F=Kx (for magnitude only)
or, mg=Kx
=150 N/m
Q2. A harmonic oscillator has a period of 0.5 sec. & amplitude of 10 cm. Find the speed of object when it passes through the equilibrium position.
Vmax           =             A.ω
                =             A× 2πf
Q3. A body of mass 0.3 Kg executes S.H.M. with a period of 25 sec. and amplitude of 4 cm. Calculate the max. velocity, acceleration & K.E.
m= 0.3 Kg, T=25 sec., A=4cm .

Q4. A small body of mass 0.1 Kg is undergoing S.H.M. of amplitude 1.0 m and period 0.2 sec (a) what is the maximum value of force acting on it?, (b) If the oscillations are produced by a spring, what is the constant of spring?
Q5. Two springs having force constants k1 & k2 respectively are attached to a mass and two fixed supports as shown. If the surfaces are frictionless, find the frequency of oscillation.
 Total restoring force,
F             =             -k1x-k2x
F             =             -k1x-k2x
ma         =             -(k1+k2)x

Q6. Show that if a uniform stick of length 'l' is mounted so as to rotate about a horizontal axis perpendicular to the stick and at a distance'd' from the centre of mass. The period has a minimum value when d=0.289 l.
General, M.I. = mK², where K= radius of gyration.

M.I. of different surfaces about c.g.
Thin ring/hollow cylinder
Hollow sphere
Solid sphere

d = 0.289l   proved.

Q7. A body executing S.H.M of amplitude 10cm & frequency 10 vibrations/sec. Calculate its acceleration after 0.015 sec.

Q8. At a time when the displacement is half the amplitude what fraction of the total energy is kinetic & what fraction is potential energy. in S.H.M ?At what displacement is the energy half K.E. & half P.E.
Q9. Uniform circular disc of radius R oscillates in a vertical pane about horizontal axis. Find the distance of the axis of rotation from the centre of which the period is minimum. What is the value of the period?
Q10. A particle executing S.H.M. of amplitude 'A' along the X-axis. At t=0, the position of the particle is  and it moves along the positive X-direction. Find the phase constant δ. If the equation is written as x=sin(ωt+δ)


                The disturbance which travels onward through the medium due to the periodic motion of particle about their mean position is called wave motion. Thus wave transfers energy from one place to another place without any bulk transformation of intervening particles of the medium, so wave is the mode of transfer of energy.
                According to the medium required or not required wave is categorized into two types.

Mechanical wave

The wave which require material medium to travel or propagate onward is called mechanical wave for e.g. sound wave.

Electromagnetic wave

The wave which do not require material medium to travel or propagate onward is called electromagnetic wave for e.g. radio wave, light wave etc.
According to the vibrations of particles wave motion is of two types


 Transverse wave motion:-

The wave motion in which particles of medium oscillates in perpendicularly to the propagation of wave is called transverse wave motion. Therefore it is in the form of crest and trough.

Longitudinal wave motion:

                The wave motion in which particles of the medium vibrates parallel to the direction of propagation of wave is called longitudinal wave motion. Therefore it is in the form of compression & rarefraction. It is also called as Pressure wave.

Progressive wave:

                The wave which travels onward through the medium in a given direction without alternation or with constant amplitude is called progressive wave. It may be transverse or longitudinal wave. In such wave all particles vibrate with same amplitude, the only difference is that they have different instant of time.
Consider a particle at 'O' executes periodic motion then, if 'y' be the displacement of particle then equation for displacement,
Let 'P' be the point at a distance 'x' from origin,
In one complete oscillation the phase is 2π or wave covers the distance λ.
when, path difference is λ then phase difference is 2π
when path difference is 1 then phase difference is 2π/λ
when path difference is x then phase difference is  
path difference between O & P will be
If the wave travels in –ve X direction then,
y=Asin (ωt+kx)

Wave velocity & particle velocity

                The rate of change of wave displacement w.r.t. time is called wave velocity (phase velocity). It is denoted by 'u' and given by,
We have wave equation for progressive wave travelling in +ve direction,
Now, differentiating w.r.t. time we get,
For given wave ωt-kx=constant
Now, differentiating w.r.t. time we get,
wave velocity = wavelength × frequency
The rate of change of displacement of the particle with time is called particle velocity. It is denoted by 'v' & given by,
but, y=Asin(ωt-k) ….(iv)
Differentiating w.r.t. x
From eqn. (v) & (vi) we get,
Particle velocity = wave velocity × (-slope displacement curve)

Intensity of wave:-

                The flow of energy per unit area per unit time is called intensity of wave.

where, m=mass of particle, A=amplitude of oscillation & ω=angular frequency
                Consider a table of cross section area's' and length 'l' when the wave energy is transmitted. Let 'n' be the number of particles per unit volume then the total number of particle in volume 'sl' will be 'nsl'.

Hence, Intensity ∝ ρ
Hence, Intensity ∝ u
Hence, Intensity ∝
Hence, Intensity ∝ A²



                The  velocity of transverse wave along stretched string can be expressed in terms of tension along the string & mass per unit length of the string.
                When a jerk is given to the stretched string fixed at one end transverse wave can be produced.
                Consider an element length Δl of string which forms an arc 'PQ' of radius 'R'. The tension produced on the string can be resolved into two perpendicular components. Tcosθ along the horizontal & Tsinθ along radial direction. The horizontal components cancel each other and the radial components provide the tension.
                The resultant tension along string = Tsinθ +Tsinθ =2 Tsinθ…(i)

Energy Transmission along stretched string         

If μ be the mass per unit length (m/Δl) of string then, m=μΔ L



                When two travelling waves of same amplitude & same wavelength travel in opposite direction with same velocity superimpose with each other then the resultant wave formed is called standing wave or stationary wave.
There are certain position like 'N' where the particle vibration is minimum i.e. particle doesn't vibrate is called node. There are certain positions like 'AN' where the particle vibration is maximum is called Anti-Node.
                The standing wave confines in certain region & cannot propagate forward.
By the principle of superposition the resultant displacement of the wave is given as, y=
This is the resultant wave equation of standing wave with amplitude
For maximum amplitude, coskx=±1
or, coskx=cosnπ  [n=0,1,2,3,….]
or, kx = nπ
For minimum amplitude, coskx=0


                In standing waves either end points are nodes or anti-nodes, particle vibrates with maximum amplitude is called resonance.
                The minimum frequency at which resonance occurs is called fundamental frequency. It is denoted by 'f0'.
For fundamental frequency, if the ends are closed i.e. if there are two nodes there is only one anti-node.

                when there are two segments in the wave,


Q1.    A wave of frequency 500 cycles per sec has a phase velocity of 350 m/s. How apart are two  points 60° out of phase ? (Ans. 0.12 m)
Q2.    A wave of frequency 500 cycles per sec has a phase velocity of 350 m/s.  What is the phase difference between two displacements of a certain point at time 0.001 sec apart? (Ans. π ͑)
Q3.    The equation of a transverse wave travelling in a rope is given by y=-10sin(0.01x-2t). Find the amplitude, frequency, velocity and wave length.
Q4.    A string of mass 2g for unit length carries progressive wave of amplitude 15 cm, frequency 60 sec-1 & speed 20m/s. Calculate a. Energy per meter length of wire, b. the rate of energy propagation in the wave.
(Ans. 6.39 J/s, 0.031 J) 
Q5.    Two waves are simultaneously passing through a string. The equation of waves are y1=A1sink(x-vt), y2= A2sink(x-vt+x0), where the wave no k=6.28 cm-1 & x0=1.50 cm, the amplitudes are A1=50mm & A2=40mm.



                The distance by the light through a certain distance for a certain time in a certain medium is called geometrical path.
                The distance travelled by the light in vacuum through a certain distance for the same time in which it travel in medium is called optical path.
                Suppose light travels through the medium of refractive index 'μ' with velocity 'v' for time't' then the geometrical path will be,
                If 'c' be the velocity of light in vacuum for time 't' then distance travelled by light,
Optical path = refractive index of medium times the geometrical path.

Interference of light waves

                The source of light distribute its energy uniformly in all directions if there are two sources which distributes energy of same wavelength & frequency the energy distribution will not be uniform. The non uniform distribution of energy due to the superposition of two light waves of same amplitude and frequency is called interference of light waves.
                At some point energy will be maximum called constructive interference & at some places energy will be minimum called destructive interference.

Coherent source

                The sources are said to be coherent if they produce the light wave of same amplitude & frequency.
For the sustain interference following conditions are to be satisfied:-
1)     Two sources should be coherent
2)     Two sources emit light wave continuously.
3)     They should be narrow sources
4)     Sources should be close to each other
5)     The distance between the sources and screen should be far
6)     Sources should be monochromatic

Condition for interference maxima &  minima

                Consider two point sources S1 & S2 which produces the interference fringes. Let 'P' be the point where we have to find the interference fringe, which is either maxima or minima.
                Let y1 = asinωt be the wave produced by S1 & y2= asin(ωt+δ) be the wave produced by S2.
The resultant displacement of waves by superposition principle can be written as,
y    =    y1+y2
      =    asinωt+ asin(ωt+δ)
      =    a[sinωt+ sinωt.cosδ+cosωt.sinδ]
      =    A[sinωt(1+cosδ)+cosωt.sinδ]….(i)
Let               a(1+cosδ)   =    Acosθ …..(ii)
                                asinδ     =    Asinθ …….(iii)
then, y  =    A[sinωt.cosθ+cosωt.sinθ]    
                =    Asin(ωt+θ)….(iv)
, which is the resultant wave equation of the resultant wave.
Squaring & adding (ii) & (iv)
         a²sin²δ+a²+a²cosδ+ a²cos² δ=A²
or,    2a²+2 a²cosδ=A²
or,    2a²(1+cosδ)=A²
         since, intensity of the wave is directly proportional to the square of amplitude, we can write,
The intensity will be maximum if =1

for 2π phase difference he path difference will be λ
The point 'P' will correspond to maximum or bright fringe if the path diff = nλ
for 2π phase difference path difference = λ

Thin films:
                Thin films is an optical medium where thickness is order of  wavelength of light in visible region (400 nm t 760 nm). It may be the soap bubble, glass or air enclosed between two transparent glass plate whose thickness is 0.5μ m to 10μm.
*Interference due to thin films
*Interference due to reflection on thin film
                Let AB be the incident light incident in upper surface, XY of the thin film of thickness't' and refractive index 'μ '. At point B, most of the part transmitted along BC and a point of it is reflected along BF.
Also, at point 'C', most part is transmitted along CN and a point of it is reflected along CD. Similarly at point 'D' most part transmitted along DC and a part of it is reflected along 'DE'.
                Only the light wave reflected from point 'B' and 'C' are of appreciable strength.
Now, the path difference between BF and DG = BC +CD-BN
Optical path difference = μ(BC+CD)-BN…(i)
Now, from fig. in Δ BCM,

from, (iv) & (v)
BN = 2tanr.sini….(vi)
At 'B' reflection takes place from the surface of denser medium, therefore the additional path difference λ/2 should be added   [At reflection the wave losses half of the wave]
for minima or dark fringe,
path difference = (2n+1) λ/2    [n=0,1,2,….]

Newton's Ring:
                Let 'S' be the source of monochromatic light. Light rays form 'S' are made parallel by lens L. These parallel rays are incident on glass plate 'G', kept 45° with horizontal. A part of these rays fall normally on plane convex lens 'P' kept on glass plate 'AB'. The Plano-convex  lens and glass plate AB create air film of variable thickness.
                The light ray reflected form upper part of thin air film and lower part of thin film interfere to produce interference fringes as from circle(ring). Newton's rings are the fringes of equal thickness.
Let 'R' be the radius of curvature of Plano-convex lens, 't' be the thickness of air film for which radius of nth ring will be 'rn'.
                For the interfere due to reflection on thin film, the path diff=2μtr+λ /2
For air film, μ = 1
for normal incidence, r=0
∴ path diff=2t+λ /2                                                                                                                     
for bright fringe path difference = nλ                                                                                       
∴ 2t+λ /2 = nλ     [n=0,1,2,….]                                                                                  
2t=nλ -λ /2
2t = λ/2 (2n-1)….(i)
for dark fringe, path diff = ((2n+1) λ /2
∴ 2t+λ/2 = (2n+1) λ /2
2t = nλ                                                                                                                                                 
from fig, in Δ PNM                                                                                                        
PM² =PN² +NM²                                                                                                                                                      
R²=rn² +(R-t)²
R²=rn² + R²-2Rt+t²
                but, the radius of curvature is very  greater then thickness of air film.
→rn² =2RT
→2t= rn²/R=Dn² /R
Where, Dn= diameter of an fringe.
for bright fringe, eqn. (i) & (iii) must be equal
If dm be the diameter of mth ring then
Now, subtracting (v) fro (iv)
                If we know the diameter of nth and nth ring, we can determine the wavelength of monochromatic light.

 Diffraction:- It is the phenomenon of bending of light round the sharp corner and spreading the light into the geometrical shadow region. As a result of diffraction, maxima and minima of light intensities are found which has unequal intensities. Diffraction is the result of superposing of waves, from infinite of coherent sources on the same wave front, after the obstacle has distorted the wave front.
There are two types of diffraction,
a.       Fresnel diffraction :-
b.       Fraunhoffer diffraction :-

TO observe the diffraction phenomenon the size of the obstacle must be of the order of the wavelength of the waves.

Fresnel diffraction :- It involves spherical wave fronts. If either source or screen or both are at a finite distance from the diffracting device , the diffraction is called Fresnel diffraction and the pattern is shadow of the diffracting device modified by diffraction effects. Diffraction at a straight edge, narrow wire or small opaque disc is familiar examples of this type. It explain diffraction in terms of  “half period zones” , the area of which is independent of the order of zone.

Fraunhoffer diffraction :-
It is the diffraction when both source and screen are effectively at infinite distance from the diffracting device and pattern is the image of source modified by diffraction effects. Diffraction at single slit, double slit and diffraction grating can be cited as examples of this type. It follows that fraunhoffer diffraction is an important special case of Fresnel diffraction.