Light consists of very Tiny,
Elastic, Rigid particles known as
These corpuscles on emission from
the source of light Corpuscular”.
When these particles enter travel in
straight line with high velocity the
eyes, they produce image of the object or sensation of vision.
Reflection and Corpuscles of
different colours have different sizes refraction of light are because of repulsive
and attractive forces respectively.
Velocity of light in denser medium
is less …Newton unable to explain
Greater than velocity of light in
rare medium.
Newton unable to explain simultaneous
reflection and refraction Scientist
unable to find out
polarization and interference
concept. relation between size and colour at larger level. At nano level its true.
These waves are emitted by the
Light is propagated in form of
waves.
source of light and travel
instraight lines with a uniform velocity
When light enter our eyes it creates an¨through ahomogeneous medium. ¨optical impression onthe retina. Hence, we get the sensation
of light. Light¨Different colours are due to different wavelengths of
lightwaves. waves are mechanical waves. For propagation of thesewaves, a
hypothetical medium called “luminiferous ether”is present everywhere. Light
travels through ether in formof waves.
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The phenomena¨6. like reflection, refraction,polarization,
simultaneous reflection andrefraction, total internal reflection,
diffractionetc can be According to
Huygens’ theory¨successfully
explained with thistheory. the speed oflight in denser medium is less than the
speedof light in rarer medium. This conclusion is inperfect agreement with the
experimentalfindings.
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The existence of so called¨7. Rectilinear¨luminiferous ether,assumed by Huygens, was not confirmed.
propagation was not explained bythe theory. It was then justified by This theory could not¨ Diffraction was explained much later.¨Fresnel. explain photoelectricemission.
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It can be defined as the¨8. locus of all the pointsof the medium to which
the wave reachessimultaneously, so that all the points are inthe same phase.
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9. Spherical Wave front :-
Consider a point sourceof light S. The light¨ waves emitted by it travelin all possible directions. If c
is the velocity oflight, then after time t each wave will reach thesurface of a
sphere of radius ct with centre S.This Spherical surface is called the
sphericalwave front at time t.
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At a very large distance¨10. from the pointsource, the spherical wave
front is so large thata small part of it is almost plane. This part iscalled
plane wave front.
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If the source of light is linear, i.e. a slit, itproduces a cylindrical
wave front.¨11.
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12. A perpendicular drawn to the
surface of thewave front at any point is¨ called a wavenormal. This is in the direction of thepropagation
of The direction in which the light
travels iscalled a¨light
at that point. ray of light. A wave normal is same asa ray of light.
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¨13.
Every point on a wave front acts as asecondary source of light, sending
outsecondary waves. The envelop of all thesesecondary waves, at any If the nature¨later instant, givesthe new wave front at that instant. of
the wave front at any instantis known, we can determine the nature andthe
position of the wave front at any laterinstant by Huygens’ construction, based
onHuygens’ principle.
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1. Consider a¨14. 2.¨known position of a sphericalwave front, at time t = 0 as
PQRS According to Huygens’ principle, as soon asthe wave front is formed, every
point on thiswave front will act as a secondary source, andwill 3. To determine the position of the¨start emitting secondary waves. wave frontafter a time t, we
have to draw spheres withevery point on the surface PQRS as centre andradius
equal to ct, where c is the velocity oflight.
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4. These spheres will represent secondarywave¨15. 5. The¨fronts. Draw a tangential surfaceP’Q’R’S’ to these spheres.
surface P’Q’R’S’ represents theposition of the wave front after time t. It
isalso a spherical wave front.
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Consider a known¨16. According to¨position of a plane wave front, at timet = 0 as PQRS
Huygens’ principle, as soon as the wave frontis formed, every point on this
wave front will act as asecondary source, and will start emitting To determine the position of the wave front
after a¨secondary waves. time t,we have to
draw spheres with every point on the surfacePQRS as These¨centre and radius equal to ct, where c is thevelocity of
light. spheres will represent secondary wave fronts. Draw atangential
surface The surface P’Q’R’S’ represents
the position¨P’Q’R’S’ to these spheres. of the
wavefront after time t. It is also a plane wave front.
· 17. Thank You