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Telephone :

PABX : 9661920-73/4980

DEPT. OF APPLIED PHYSICS, ELECTRONICS &

COMMUNICATION ENGINEERING

UNIVERSITY OF DHAKA

DHAKA-1000, BANGLADESH

FAX: 880-2-8615583

E-MAIL: [email protected]

Ref. No............................ Dated, the.

In case of any query or suggestion please contact with Sazzad, Lecturer, APECE, DU (url: sazzadmsi.webs.com)

The Nature of Light:Light, which exists in tiny packets called photons, exhibits properties of both wave and particles. Four primaryproperties of light are

(I) Intensity,(II) Frequency or wavelength,(III) Polarization, and(IV) Phase.

[Ref.: http://en.wikipedia.org/wiki/Light](1) Ray:In optics, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light through an

optical system. Ray tracing uses approximate solutions to Maxwells equations that are valid as long as the lightwaves propagate through and around objects whose dimensions are much greater than the light's wavelength. Raytheory does not describe phenomena such as interference and diffraction, which require wave theory involving thephase of the wave.A light ray is a line or curve that is perpendicular to the light's wavefronts. Light rays bend at the interface betweentwo dissimilar media and may be curved in a medium in which the refractive index changes.[Ref.: http://en.wikipedia.org/wiki/Ray_(optics)](2) Photons:There are a number of phenomena that the wave model of light cannot explain. But photons are governed byquantum mechanics and will exhibit wave-particle duality, that is, they exhibit properties of both waves and particles.A photon is an elementary particle, the quantum of the electromagnetic field and the basic unit of light and all otherforms of electromagnetic radiation. It is also the force carrier for the electromagnetic force.

[Ref.: http://en.wikipedia.org/wiki/Photons](3) Electromagnetic Waves:

Fig.: The structure of an electromagnetic wave.

Y

Z

X

Y

Z

X

Electric field

Magnetic field

Time

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Telephone :

PABX : 9661920-73/4980



UNIVERSITY OF DHAKA


FAX: 880-2-8615583


Ref. No............................ Dated, the.

In case of any query or suggestion please contact with Sazzad, Lecturer, APECE, DU (url: sazzadmsi.webs.com)

Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye, about 380-750nm. An electromagnetic wave consists of two fields an electric field and a magnetic field. Both of these fieldshave a direction and a strength or amplitude. Within the electromagnetic wave the two fields are oriented at precisely90 to one another. The fields move at the speed of light in a direction at 90 to both of them. In three dimensions the electric field to be oriented on the y-axis, the magnetic field on the x-axis, and the direction of travel would then bealong the z-direction.As the electromagnetic wave moves the fields oscillate in direction and in strength. The field might start at amaximum in one direction, decay to a zero and then build up in the other direction until it reaches a maximum in thatother direction. The field strength changes sinusoidally.The two fields oscillate in phase. That is, the electric and magnetic fields reach their peaks and their nulls at exactlythe same time and place. The rate of oscillation is the frequency of the wave. The distance traveled during one periodof oscillation is the wavelength.So, a particle or ray of light travels through space as two interlocking force fields electric and magnetic. These fieldscentre on a point over time and this is of course a line in the direction of propagation, and decay exponentially as itmoves away from that point or line.[Ref.: Understanding Optical Communications, Harry J.R. Dutton]

Electromagnetic Waves Generation:

A time-varying

E-field generates a

B -field, which is everywhere perpendicular to the direction in which

E-fieldchanges.

In the same way, a time-varying

B -field generates an

E-field, which is everywhere perpendicular to the direction in

which

B -field changes.Now, consider a charge that is somehow caused to accelerate from rest.

When the charge is motionless, it has associated with it a constant radial

E-field extending in all directions.

At the instant the charge begins to move, the

E-field is altered in the vicinity of the charge, and this alterationpropagates out into space at some finite speed. This time-varying electric field induces a magnetic field.

If the charges velocity is constant, the rate-of-change of the

E-field is constant.

But as the charge is accelerating,t

E

is itself not constant, so the induced

B -field is time-dependent. This time-

varying magnetic field generates an electric field.

The process continues with

E and

B coupled in the form of a pulse. As one field changes, it generates a new fieldthat extends a bit farther, and the pulse moves out from one point to the next through space.

Hence, the

E- and

B -fields can be considered as two aspects of a single physical phenomenon, theelectromagnetic field, whose source is an accelerating charge.[Ref.: Optics, Eugene Hecht]

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Telephone :

PABX : 9661920-73/4980



UNIVERSITY OF DHAKA


FAX: 880-2-8615583


Ref. No............................ Dated, the.

In case of any query or suggestion please contact with Sazzad Lecturer APECE DU (url: sazzadmsi webs com)

Spherical Waves:

Fig.: Energy distribution emanating from a point source with a spherical wave front.The wave front of a point source is a spherical wave.Let a point source be located at the origin and let it oscillate with respect to time as e-jt.The wave front emanating from the point source takes r/v seconds to reach a sphere of radius r, v being the speed ofpropagation. The field on the sphere is given by

)(

)/(

)(

)(),(

krtj

vrtj

erE

erErtE

=

=

where, k=/v=2/ is the propagation constant.For a plane wave front the energy density of the electromagnetic wave is |E|2, being the dielectric constant of themedium. If the spheres radius is large compared to the dimensions of the selected surface area, the sphericalsurface can be approximated by a plane.The point source is an omnidirectional emitter and the light energy is constantly flowing out without collectinganywhere. Hence the total energy passing through any arbitrary sphere centered at the source per unit time isconstant regardless of its radius, and is given by

r

ErE

vrErW

0

22

0

)(

)(4

=

=

where E0 is the amplitude of the field at unit distance away from the source.Now the expression for the spherical wave becomes

)(0),( krtjer

ErtE =

[Ref.: Engineering Optics, Keigo Iizuka]

y

z

x

O

Point source S

|E(r)|2

Element surface ontangential plane

Element wave fronton the sphere

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