1. “DOUBLE SLIT DIFFRACTION”
PRESENTED BY
SARA
DEPARTMENT OF PHYSICS
3RD SEMESTER (B.S)

2. DIFFRACTION:
The bending of wave around an obstacle is called
diffraction.
APPLICATIONS:
Because of diffraction we can hear sound outside
the close room.
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3. INTRODUCTION:
• The double-slit experiment is a nineteenth-century
investigation into the properties of light.
• This experiment is sometimes referred to as Young's
experiment.
• The experiment belongs to a general class of
"double path" experiments, in which a wave is split
into two separate waves that later combine back
into a single wave.
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4. DOUBLE SLIT DIFFRACTION:
Young began his experiment by sending waves
of coherent light towards a barrier with two slits.
The two slits are separated by a distance d. On a
screen, a perpendicular distance L from the slits, a
series of interference fringes were viewed.
The formulas we will now develop will allow us to
determine if a point P in the interference pattern a
distance y from its center will fall into a bright zone or
into a dark zone.
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7. EXPLAINATION:
EPD represents extreme path difference, or the
effective difference in the distances the light must
travel to reach a given position on the screen from
each of the slits.
For bright fringes, this path difference must be
a multiple of a wavelength to insure constructive
interference. When this overall path difference equals
an odd-multiple of a half wavelength, then
destructive interference is insured and a dark fringe is
formed on the screen.
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8. CONDITIONS:
• Thomas Young's equation for double slit interference is
• For bright fringes (constructive interference)
• For dark fringes (destructive interference)
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where
L
y
d
EPD
'
sin  L
y
tan
mEPD  ........)3,2,1,0(m
2
)12(

 mEPD ........)3,2,1(m

9. EXAMPLE OF LASER LIGHT:
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10. LASER DIFFRACTION COMPARED TO
INTENSITY DIAGRAMS
• The pattern formed by the interference and
diffraction of coherent light is distinctly different for
a single and double slit. The single slit intensity
envelope is shown by the dashed line and that of
the double slit for a particular wavelength and slit
width is shown by the solid line. The photographs of
the single and double slit patterns produced by a
helium-neon laser show the qualitative differences
between the patterns produced
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11. LASER DIFFRACTION COMPARED TO
INTENSITY DIAGRAMS
• The number of bright maxima within the central
maximum of the single-slit pattern is influenced by
the width of the slit and the separation of the
double slits.
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