(1) The document describes an experiment to determine the thickness of a human hair using laser diffraction. Students illuminated a mounted human hair with a 650nm laser and recorded the diffraction pattern on a screen. (2) They measured the distances between fringes to calculate the hair's thickness using the diffraction equation. (3) Additionally, students measured the intensity distribution of the diffraction pattern by moving a light dependent resistor across it and recording the current readings.

1. Fresnel diffraction due to
human hair
Project on
ST.TERESA'S COLLEGE (AUTONOMOUS), ERNAKULAM
DEPARTMENT OF PHYSICS AND CENTER FOR RESEARCH
Presented By:
Malavika S (AB20PHY017)
Swetha Sudev (AB20PHYO36)
Guided by :
Dr. Mariyam Thomas

2. Aim and Objective
(i) To determine the thickness of a human hair using laser diffraction.
(ii) To determine the intensity distribution of the diffraction pattern due to the
human hair strand.

3. Diffraction
• It is the process by which a stream of light or wave is spread out as a result of
passing via a narrow area or across an edge, generally accompanied by
interference between the waveform produced.
• Two types of optical diffractions:
i. Fresnel diffraction
ii. Fraunhofer diffraction
• Condition for diffraction:
The width of the obstacle must be less than or comparable with the
wavelength of the wave.

4. Theory
• The thickness of an object can be
determined by observing the diffraction
pattern produced when a wave of
wavelength comparable to the size of the
object falls on it.
• For a bright fringe of nth order
Fringe width β = (D λ n) / d
Where λ is the wavelength,
D is the distance between the
screen and the object
and d is the thickness of the object

5. • To find the intensity distribution of the diffraction pattern, light detector resistance can
be used.
• In LDR, the resistance (R) is inversely proportional to the intensity (I’) of the incident
light.
R ∝ 1/I'
But from Ohms law, we can write
I ∝ 1/R
∴ I ∝ I'
Where I is the current measured
Let I0’ be the intensity of the central maxima and I0 be the current detected for nth order
bright fringe having intensity I’ and current I, we can write that
I/I。=I'/I'。'
Where I’/I0’ is the intensity ratio.

6. Apparatus
• To determine the thickness of human hair
I. Laser source of wavelength 650 nm
II. Human hair strand
III. Square frame
IV. Retort stand
V. Graph paper
VI. Paper pin
VII. Meter scale
• To measure the intensity distribution of the diffraction pattern
I. LDR
II. Current display unit
III. Manual linear stage

7. Procedure
 To determine the thickness of human hair
 The hair strand whose thickness is to be determined is mounted on a stand horizontally. It is
made sure that the hair is taut and straight.
 The laser pointer of wavelength (λ) 650 nm, is positioned on the retort stand such that the laser
beam strikes the mounted hair strand.
 The experimental setup is positioned such that the diffracted pattern of the hair strand falls on a
screen.
 A graph paper is fixed on the screen using paper pins.
 Record the distance between the screen and the hair strand (D).
 Trace the pattern produced by the diffraction of the hair strand onto the graph paper.
 The distance from the central maxima to the starting end and finishing end of 1st order on left
side is calculated. The average value is noted under x column in the observation table
 The above step is repeated on the right side of the central maxima and the values are recorded
in the observation table.
 The diameter of the hair strand (d) is calculated using
d = nλD / x

8. Hair strand mounted
horizontally on a frame
The laser beam is incident on
the hair strand
Observed diffraction
pattern

9. (ii) To determine the intensity distribution of the diffraction pattern due to
human hair strands.
• In the above experiment, the screen is replaced with the LDR. the LDR is
connected to the current detection output unit and is mounted on a manual linear
stage.
• The LDR is mounted manual linear stage is adjusted so that the central bright band
of the diffraction pattern falls on the LDR.
• The lateral screw is moved 1mm towards the left and the value of the current is
recorded in the given observation table. This process is repeated up to 25 mm
lateral displacements.
• Similarly, the value of the current for each lateral displacement on the right is
recorded in the observation table.
• The intensity ratio for each lateral displacement is calculated and recorded in the
observation table.

10. LDR mounted on a manual linear stage and
current detector output unit

11. Observation
 To determine the thickness of the human hair strand.
 The wavelength of the laser source = 650 nm
Mean diameter d = 0.0617 mm

12. Intensity distribution

13.  The intensity ratio vs lateral displacement is plotted

14. Conclusion
 The thickness of the hair strand is measured to be 0.0617 mm.
 From the intensity distribution graph, we can observe that the intensity of
the diffraction pattern decreases in a continuous pattern when moved away
from the central maxima.
 There is a very small increase in intensity. When these points are observed,
they are found to be the points of secondary maxima.

15. UC College Exhibition
Field trip report

16. 
As part of the 100 years Celebration of Union
Christian College, they organised a one-week-long
exhibition along with various cultural programs from
7th - 12th November 2022. On 9th November 2022,
44 students from III DC and I PG Physics went on a
field trip to UCC to visit the exhibition.

17.  We went to the zoology department, where we got to experience different terrains and
ecosystems through a setup created by the students of UCC. A vast display of preserved
species gave us an insight into how diverse our ecosystem is.

18.  Then we went to the physics department and we saw an exhibition of various
instruments that were used in ancient times to measure different parameters
and to prove certain phenomena.

19. Conclusion
 Through this trip, we got exposed to new fields apart from physics which may help
students in the future. The main motive behind this field trip was fulfilled thus we can
conclude that the field trip was successful and beneficial for the students.

20. Thank you