PHYSICS Pocket diary

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radioactivity questions download

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What is diffraction

It is a common observation with the waves of all kinds that they bend round the edge of an obstacle.

Light like other waves also bends round the corner but in comparison to sound waves small bending of light is due to very short wavelength of light which is of the order of 10^-5 cm.

This effect of bending of light beams round the corner was first discovered by Grimaldi (1618-1663).

We now define diffraction of light as phenomenon of bending of light waves around the corners and their spreading into geometrical shadow.

Fresnel first explained that the diffraction phenomenon was the result of mutual interference between the secondary wavelets from the same wavefront.

Thus we can explain diffraction phenomenon using Huygen's principle.

The diffraction phenomenon is usually divided into two classes
1. Fresnel class of diffraction phenomenon where the source of light and scteen are in general at finite distance from the diffracting aperture
2. Fraunhofer class of diffraction phenomenon where source and the screen are at the infinite distance from the aperture , this is easily achieved by placing the source of light on the focal plane of a convex lens and placing screen on focal plane of another convex lens. This class of diffraction is simple to treat and easy to observe in practice.

Solve out mechanics problems (IITJEE tips and tricks for mechanics)

Hi all here in this article i am giving some tips and tricks to solve problems in mechanics. To master problem solving skill you need to practice more and more problems as said practice makes a man perfect. Look at examples in your text books identify the steps and then try solving out problems on your own.
 
Motion in a Two dimensional Plane

1. Select a coordinate system and resolve the initial velocity vector into x and y components.
2. Find out acceleration in each direction and solve in each direction according to one rectilinear motion equation.
3. If the acceleration is in vertical direction only.Follow the techniques for solving constant-velocity problems to analyze the horizontal motion. Follow the techniques for solving constant-acceleration problems to analyze the vertical motion. The x and y motions share the same time of flight t.
4. There might be question about trajortory in the Problem ,find out the motion in x and y direction with respect to time from previous point.And then find the value of t from one equation and then put that value in another equation to find out the equation of trajactory

Motion in a Three dimensional Plane
1. Select a coordinate system and resolve the initial velocity vector into x , y and z components
2. Find out acceleration in each direction and solve in each direction according to one rectilinear motion equation.

Uniform Circular Motion
1. Draw a simple, neat diagram of the system.
2. Firstly consider the origin of the forces acting on the each object.To do this find out the field forces acting on the each object.Wherever contact in available account the contact force carefully
3. Find out the force acting on the body.The resultant force should provide the required centrepatal required for Circular motion
4. Centrepatal force=mv2/R will give the velotiy accordingly
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Coherent Sources of light

Coherent sources are those sources of light which emit continous light waves of same wavelength , same frequency and are in same phase or have constant phase difference.

For observing interference phenomenon coherence of light waves is a must.

For light waves emitted by two sources of light , to remain coherent the initial phase difference between waves should remain constant in time. If the phase difference changes continously or randomly with time then the sources are incoherent.

Two independent sources of light are not coherent and hence can not produce interference because light beam is emitted by millions of atoms radiating independently so the phase difference between waves from such sources fluctuates randomly many times per second.

The coherent sources can be obtained either by the source and obtaining its virtual image or by obtaining two virtual images of the same source. This is because any change of phase in real source will cause a simultaneous and equal change in its image.

Generally coherence in interference is obtained by two methods
(1) Division of wave front where wavefront is divided into two parts by reflection, refraction or diffraction and those two parts reunite at a small angle to produce interference as done in case of Young's double slit experiment and Fresnel's biprism experiment.
(2) Division of amplitude where amplitude of a section of wavefront is divided into two parts and reunited later to produce interference such as in case of thin films.

Laser light is almost monochromatic light with little spreading and two independent sources of laser light can produce observable interference pattern.

Magnetic field

We will now look at the properties of the magnetic field which are related to the flux and circullation of the vector field to express the basic laws of magnetic field. We already know how to represent electric field graphically and unlike any other vector field magnetic field B can be represented with the help of field lines drawn in such a way that tangent to those lines at any point concides with the direction of the magnetic field B and the density of the lines is proportional to the magnitude of the vector at a given point. we would now consider the basic laws of magnetic field....
(1) Gauss's Theorem for the field B:-
It says that "Flux of B through any closed surface is equal to zero". i.e.,

This says that field lines of vector B neither begning nor end and therefore field lines of vector B emerging from any volume closed by surface S is always equal to the number of lines entering this volume. This law also indicate the absence of magnetic charges on which field lines of vector B begin or terminate i.e., magnetic fields has no sources as charges are for electric field.

(2) Theorem of circulation of magnetic field:-
It states that " Circulation of vector B around a arbitrary contour C is equal to the product of magnetic permeability times the algebric sum of currents enveloped by the contour C".

The current is assumed to be positive if its direction is connected with the direction of the circumvention of the contour C through the right hand screw rule and is negative if it is in opposite direction. This theorem can be proved by means of Biot Savart's Law. This theorem plays same role for magnetic field as Gauss's theorem plays for electric field.

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Black body radiation

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