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DIFFRACTION
OF LIGHT
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The
bending and spreading of light waves around sharp edges or corner or
through small openings is called Diffraction of Light. |
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CONDITIONS
FOR DIFFRACTION
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| Diffraction
effect depends upon the size of obstacle. Diffraction of light takes place
if the size of obstacle is comparable to the wavelength
of light. Light waves are very small in wavelength, i.e. from 4 x 10-7 m to 7 x 10-7 m. If the size of opening or obstacle is near to this limit, only then we can observe the phenomenon of diffraction. |
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TYPES
OF DIFFRACTION
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| Diffraction of light can be divided into two classes: | |||
Fraunhoffer diffraction. Fresnel
diffraction. |
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FRAUNHOFFER
DIFFRACTION
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In
Fraunhoffer diffraction, Source
and the screen are far away from each other. Incident
wave fronts on the diffracting obstacle are plane. Diffracting
obstacle give rise to wave fronts which are also plane. Plane
diffracting wave fronts are converged by means of a convex lens to produce
diffraction pattern. |
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FRESNEL
DIFFRACTION
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| In
Fresnel diffraction, Source
and screen are not far away from each other. Incident
wave fronts are spherical. Wave
fronts leaving the obstacles are also spherical. Convex
lens is not needed to converge the spherical wave fronts. |
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DIFFRACTION
GRATING
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| A
diffraction grating is an optical device consists of a glass or polished
metal surface over which thousands of fine, equidistant,
closely spaced parallel lines are been ruled. For latest information , free computer courses and high impact notes visit : www.citycollegiate.com |
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PRINCIPLE
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| Its working principle is based on the phenomenon of diffraction. The space between lines act as slits and these slits diffract the light waves there by producing a large number of beams which interfere in such away to produce spectra. | |||
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GRATING
ELEMENT
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| Distance between two consecutive slits(lines) of a grating is called grating element. If 'a' is the separation between two slits and 'b' is the width of a slit, then grating element 'd' is given by; | |||
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d
= a + b
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d
= length of grating/no. of lines
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DETERMINATION
OF
WAVE LENGTH OF LIGHT BY DIFFRACTION GRATING |
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| Consider two parallel rays of light r1 and r2 falling on a grating .After passing through slits they are diffracted and brought to focus on a screen at point B by using a convex lens.. Draw a perpendicular 'pq' from P on r1. Ray (1) covers a distance "rq" more than ray (2). | |||
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Now
consider right angled Drpq
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rq/rp
= sinq
Or rq= rp sinq But rp = (a+b) (a+ b ) = d (grating element) rq = dsinq -----(1) |
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Where
rq= path difference of r1 and r2.
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We know that for constructive interference at point 'B', path difference between r1 and r2 will be 0, l, 2l, 3ll, …….,mll. Therefore, rq = ml for bright point Putting this value in equation (1), we get ml = dsinq where m = order This equation is called "grating equation" and is used to determine he wavelength of light. |
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CONCLUSION
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'm' is called the order of grating and it is the number
of bright or dark fringe obtained on the screen. For m=0 , q=0, central bright maxima of zeroth order. m=1 , q=q1, 1st order bright maxima (path difference = l) m=2 , q=q2, 2nd order bright maxima (path difference = 2l) With the increase in 'm', fringes of decreasing width and less brightness are obtained. No order of line is possible at q>90o. |
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CHARACTERISTICS
OF
GRATING SPECTRA |
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Spectra of different orders are obtained symmetrically on both sides of
zeroth order image. Spectral
lines are almost straight and quite sharp. Spectral
colors are in the order. www.citycollegiate.com The spectral
lines are more and more dispersed as we go to higher orders. Most
of the incident intensity goes to zeroth order and rest of it
is distributed among the other orders. |
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| For latest information , free computer courses and high impact notes visit : www.citycollegiate.com | |||