Using the laser source one can determine the wavelength of the laser source thickness of wire and diameter of circular aperture

1. FRESNEL
EXPERIMENTS USING LASER
Associate Professor in Physics
SARAH TUCKER COLLEGE [AUTONOMOUS]
TIRUNELVELI-627007

2. Determination of diameter of a thin wire
Aim:
To determine the diameter of a thin wire using the phenomenon of
diffraction.
Apparatus required:
A helium-neon laser, thin wire mounted in the holder and a screen.
Formula:
d =
𝝀𝑫
𝑿
metre
Where,
λ- Wave length of He-Ne laser in m (6328Å)
d- Diameter of the thin wire in m
D- Distance between the screen and the wire in m
X- Width of the central maxima in m

3. Thin wire

4. METHOD
Illuminate the wire
with the laser beam as
shown in the figure and
observe the diffraction
pattern on the screen.
Measure the distance D of
the screen from the wire
and the width of the
central maxima X.

6. Distance of
the wire from
Screen D (cm)
Width of central
maxima (cm)
d (m) x 10 -5
130 1.2 6.855
140 1.3 6.815
150 1.4 6.780
160 1.5 6.749
170 1.6 6.723
180 1.8 6.328
190 1.9 6.328
200 2.0 6.328

7. CALCULATION
Diametre of the thin wire;
d=
λ 𝐷
𝑋
metre
Wavelength of helium-neon laser
λ= 6328 x 10 -10 metre
D=130x10 -2 metre; X = 1.2x10 -2 metre
d=
6328 x 10 −10 x1.3
1.2x10 −2
metre
d= 6.855x10 -5m
Result : The diameter of the thin wire = 6.855x10 -5m

8. DETERMINATION OF THE DIAMETER OF A CIRCULAR
APERTURE
Aim:
To determine the diameter of the given
circular aperture using Fresnel diffraction
Apparatus required:
A He-Ne laser, a circular aperture or pin hole
of 0.1 to 0.5 mm diameter, a screen and a
measuring tape.

9. FORMULA
To find the diameter of the circular aperture
d=
𝝀𝒏𝒃
𝑿 𝒏
m
• Xn - Radius of the nth dark ring in m.
• n - Order of the ring.
• b- Distance between the screen and aperture in m.
• d - Diameter of the circular aperture in m.
• λ- Wavelength of the He-Ne laser (6328 Å)

10. THEORY
When the laser beam passes through a pin hole , the
distribution of light shows a bright maximum surrounded by
a number of secondary minima of decreasing intensity. This
is the diffraction pattern due to circular aperture.
Suppose the diameter of the circular aperture is d and
the screen is placed at a distance b from the aperture, then
the radius of the nth dark ring X n is given by
X n=
𝝀𝒏𝒃
𝒅
Using the formula, d=
𝝀𝒏𝒃
𝑿 𝒏
we can determine the diameter‘d’ of the circular aperture.

11. EXPERIMENTAL SETUP
 Mount the pin hole on the stand and adjust the laser beam to fall on
the hole. Allow the out coming beam to fall on the screen.
 Screen should be about 2 meters away from the aperture to have
clarity of nth dark ring (say 4thring).
 Measure the distance ‘b’ of the screen from the aperture and
calculate the diameter ‘d’ of the aperture using the formula
 Here λ is the wavelength of laser light which is 6328 Å
Note: Keep the aperture about 50 cm away from laser and about 2 to 2.5
m away from the screen .

12. b (cm) n Xn (cm) d (m)
170
1 0.9 1.195
2 1.4 1.537
3 1.8 1.793
4 2.5 1.721
5 3.0 1.793
180
1 1.0 1.139
2 1.6 1.424
3 2.1 1.627
4 2.8 1.627
5 3.6 1.582
190
1 0.9 1.336
2 1.5 1.603
3 2.0 1.803
4 2.6 1.850
5 3.1 1.939

13. d=
𝝀𝒏𝒃
𝑿 𝒏
m
Wavelength, λ= 6328 x 10 -10 metre
b=170x10 -2 m; n=1; Xn=0.9x10 -2 m
d=
6328 x 10 −10 x1.7
0.9 x10−2
metre
d= 1.195 x 10 -4m
CALCULATION
Result: The diameter of the circular aperture d= 1.195 x 10 -4m

14. REFERENCE
 II MSc Non Electronics Lab Manuel – Department of Physics Sarah Tucker
College ( Autonomous) , Tirunelveli -7
 Google Images
 PPT - https://www.slideshare.net/shoaib4700/laser-diffrection
 YouTube Links
 https://www.youtube.com/watch?v=Fs8TQzDTHNA
 https://www.youtube.com/watch?v=rmg1XyOSAk0&t=168s
 https://www.youtube.com/watch?v=Kj5_O4sxRAU&t=417s
 https://www.youtube.com/watch?v=nWbXOThQ8y4