Diffraction and
interference of light
26/5/2021
Name: Abdelrahman Aly Shehata
Id: 120200250
Section: 6
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1. Abstract:
Electromagnetic radiation propagates as a wave, and as such can exhibit
interference and diffraction. This is most strikingly seen with laser light, where
light shining on a piece of paper looks speckled (with light and dark spots) rather
than evenly illuminated, and where light shining through a small hole makes a
pattern of bright and dark spots rather than the single spot you might expect from
your everyday experiences with light. In this lab we will use laser light to
investigate the phenomena of interference and diffraction
2. The aim of the experiment:
- Observe the diffraction pattern of plane waves through a single slit and
investigate the relation between the slit width and the diffraction angle.
- Monitor interference patterns by multiple slits and examine how the patterns
depend on the number of slits to understand the principle that describe the
phenomenon of diffraction and interference.
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3. Theory:
When a monochromatic and coherent light passes through a single or double slit, it creates a
diffraction/interference pattern on a screen placed beyond the slits. The pattern formed is
because of the superposition of the waves coming from the slit (or two slits). The position on the
screen directly opposite the slits is defined to have location y = 0. Other positions on the screen
are characterized by their distance y away from this origin. Alternatively, a position on the
screen is characterized by an angle θ formed by the line from the slits to this position, relative to
the perpendicular line.
Single-slit diffraction:
Diffraction pattern formed by a single will have a wide and bright pattern at the center with
alternate dark and bright fringes with diminishing intensity on both sides. The pattern is formed
is because of the superposition of the waves coming from all points in the slit. A single slit with
slit width D will produce dark regions on the screen at positions where the following destructive
interference criterion is satisfied. D sinθ = m λ, where n is a non-zero integer. Again, for our
experiments sin θ  tan θ = y/L. Substituting y/L for sinθ in the above equation,
we get Dy/L=mλ .
Double slit Interference:
Interference pattern due to a double slit will have dark and bright fringes due to destructive and
constructive interference of the waves coming from the two slits. When two slits separated by a
distance d produces bright spots on the screen centered at positions where the following
constructive interference criterion is satisfied: d sin θ =m λ, where n is an integer. For the
experiments we will be doing, the angle θ is less than 10 degrees, and sin θ  tan θ = y/L.
Substituting y/L for sin θ in the above equation, we get dy/L=mλ .
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4. Setup and Procedure:
Apparatus:
1- He-Ne laser.
2- Optical bench with slit holder.
3- Set of slits.
4- Screen.
Procedure:
- Attach the single slight to the slit holder mounted on the optical bench. Set the
laser, the optical bench and the viewing screen so that the laser beam is parallel to
the bench and perpendicular to the slit.
- Measure distance L between the slit and the screen, L should be larger than 1m.
(Fraunhofer Diffraction)
- From the diffraction pattern, locate the position of the central maximum and
those of dark fringes up to m=3
- From the equation 𝒂 𝒔𝒊𝒏𝜽 = 𝒎𝝀 find the width of the slit.
- Repeat the same steps as single slit and calculate the slit width from 𝒅 𝒔𝒊𝒏𝜽 =
𝒎𝝀
5. References:
. 1.https://ocw.mit.edu/courses/physics/8-02-physics-ii-electricity-and-magnetism-spring-2007/e
xperiments/experiment9.pdf
2.http://depthome.brooklyn.cuny.edu/physics/lab/phy2/newlabs/Interference-and-diffraction-ver1.pdf
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