Young's double-slit experiment demonstrated the wave-like properties of light. Coherent light was passed through two slits and an interference pattern of bright and dark fringes was observed on a screen. The path difference between light passing through each slit determines whether constructive or destructive interference occurs at a given point. Thomas Young developed an equation relating the path difference to the spacing and location of the interference fringes. Laser diffraction through single and double slits was compared, showing qualitative differences between the patterns.

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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