Single Slit Diffraction Experiment
Single Slit Diffraction Experiment
Single Slit Diffraction Experiment
Wavelength of Light
Using Single Slit
THEORY BEHIND THE
EXPERIMENT
(The Classical and the Modern Approach)
HISTORY OF LIGHT
Points to be covered:
Newton’s Corpuscular Theory of Light
Inability to Explain New Observations
Huygens to the Rescue
Maxwell comes with a Surprise.
THE CORPUSCULAR THEORY(1704)
Perhaps the most simple Model of light to ever be proposed.
It was proposed by Isaac Newton (In his book, Opticks, he wrote “Are not the
ray of light very small bodies emitted from shining substance?”)
The postulates of the theory:
• Light consists of very small particles.
• The particles are very small in dimensions, thus, collision between particles is a rare
event.
• Particles of different sizes give rise to the sensation of different colors at the retina of
the eye.
• Particles of different sizes refract differently.
The success(s) of the theory:
sin 𝜃1 𝑣2
=
sin 𝜃2 𝑣1
NOTE: Although, the model satisfies the Snell’s Law, it predicts that if the ray
moves towards the normal(i.e., if the refraction occurs at a denser medium), its
speed would become higher, which is inconsistent with experimental
observations.
The two experimental facts which led to the early belief in the Corpuscular
Model are:
1. Rectilinear propagation of light leading to formation of sharp
shadows.
2. Light could propagate through vacuum.
NOTE: The domain of physics in which light is assumed to travel in straight lines
is known as geometrical optics, which can easily be explained on the basis of
the Corpuscular Theory of Light.
INABILITY TO EXPLAIN NEW
OBSERVATIONS
Some weird observations and clever experiments:
1. Careful experiments showed that shadows are not perfectly sharp.
2. Light getting polarized.
3. Formation of thin films on soap.
4. Young’s Double Slit Experiment.(A revolutionary experiment)
Around 1665, Francesco Grimaldi, was probably the first person to observe
the phenomenon of diffraction od white light as it passed through small
apertures. He was quoted:
“light is a fluid that exhibits wave-like motion”
The better thing to note about this point is that, this observation was much
before Newton’s Model(1704).
In 1678(again, much before Newton), Huygens put forward the wave model
of light. Using this model, he was successful in explaining the laws of
reflection and refraction.
However, so compelling was Newton’s authority that it is said that people
around Newton had faith in his corpuscular theory more than Newton
himself.
The reason behind calling Young’s Double Slit Experiment a ‘revolutionary’
one has its own reputation. No one believed in Huygens wave model of light
until Thomas Young performed the famous interference experiment, whose
results could only be explained on the basis of the wave model of light.
In 1802, Young gave satisfactory explanation for the formation of Newton’s
Rings. In 1816, Fresnel explained the diffraction phenomenon using wave
theory. In 1808, Malus observed polarization of light.
In the second quarter of the 19th century the wave model of light seemed to
be very well-established.
Postulates of Huygens Wave Theory of light:
ℎ
Δ𝑥. Δ𝑝𝑥 ≥
4𝜋
Where, ‘x’ is the momentum and 𝑝𝑥 is the momentum component along the
‘x’ direction.
• Here is video by Prof. Walter Lewin Explaining the Heisenberg’s Uncertainty
Principle using a single slit:
What essentially was done in this experiment is the same thing that we will be
doing in ours, except that our slit will be having a constant width.
Thus, we can see that diffraction is nothing but a direct consequence of
Heisenberg’s Uncertainty Principle.
LASERS
(SPECIAL FOCUS ON HE-NE LASER)
Introduction
Specifications of the He-Ne Laser
INTRODUCTION
A laser is a device that emits light through a process of optical
amplification based on the stimulated emission of electromagnetic
radiation. The term "laser" originated as an acronym for:
“Light Amplification by Stimulated Emission of Radiation“.
The first laser was built in 1960 by Theodore H. Maiman at Hughes Research
Laboratories, based on theoretical work by Charles Hard Townes and Arthur
Leonard Schawlow. A laser differs from other sources of light in that it emits
light coherently. Spatial coherence allows a laser to be focused to a tight
spot, enabling applications such as laser cutting and lithography. Spatial
coherence also allows a laser beam to stay narrow over great distances
(collimation), enabling applications such as laser pointers.
SPECIFICATIONS OF HE-NE LASER
• Type: Gas Laser
• Gain Medium: Helium and Neon (10:1)
• Operational Wavelength: 632.8 nm.
• Spectrum Position: Red Part of the Visible Spectrum
PROCEDURE
&
CALCULATIONS
(The Book and the Experience)
LEAST COUNT OF TRAVELLING
MICROSCOPE
• Calculate the total no. of divisions in 1 cm. If there are ‘m’ divisions in 1 cm,
then:
m div = 1 cm
Thus,1 div = (1/m) cm
• If there are ‘n’ divisions in the Vernier Scale, it means that:
n VSD = (1/m) cm
Thus, 1 VSD = (1/mn) cm
• If the Main Scale reading is at ‘M’ and the ‘Nth’ VSD is coinciding with the
MSD, then the reading is given as:
Reading = (M)+(N x VSD)
THE SLIT WIDTH
First, set the travelling microscope on a flat surface. Lay a stand for the slit in
front of the microscope. On top of it, put a grating stand and place the slit
on the grating stand.
Initially, place the stand of for placing the grating immediately in front of the
microscope and then slowly take it a little away. (It should be done so that
we do not have to keep on searching the slit through the microscope once
we have started to move it to take the readings.)
We must take the measurements in the same direction of the propagation of
the microscope. (we must make sure that the cross-hair must be vertical,
that helps a lot.)
Starting from one side, stop the microscope when the cross hair just overlaps
the first edge that we meet. Take the reading. Now, moving the microscope
‘through the slit width’ towards the other edge and stop at it. Now take the
other reading. Subtract the readings from one another and take the
absolute value.
THE BOOK
(WHAT THE BOOKS TELLS US TO DO*)
Adjust the position of He-Ne source so that the axis of the laser tube is
horizontal. Make sure the optical bench is in level.
Place the slit in front of He-Ne laser source on an upright and make it
vertical.
Illuminate the slit with the laser beam.
Place the screen S at a large distance from the slit (1 to 3m) and adjust it’s
position so as to get a sharp diffraction pattern.
Do NOT wait for the experiment to end and then you check whether the
laser beam is horizontal and not tilted.
Laser beam should not be looked upon directly with naked eye.(This one
goes unsaid)
Once a sharp diffraction pattern is achieved, the set-up should not be
disturbed. Please take very good care about this part because once we get
the pattern(which is tedious in itself), its very easy to mess it up.
REFERENCES
• Ghatak, Ajoy, Optics, McGraw Hill Education (India) pvt. Ltd. , 5th ed,
2015.
• Sanon, Geeta, BSc Practical Physics, India
• https://www.google.co.in
• https://en.wikipedia.org/wiki/Lasers