4. DOUBLE SLIT EXPERIMENT
Thomas Young (1773-1829) was a British Polymath and Physician and was born in United
Kingdom. At the age of fourteen Young had learned Greek, Latin, Arabic, Persian, Turkish,
Italian, French and etc. He was known for the Light, Young’s Temperament, Young’s
Modules, Young’s Interference Experiment and many more.
HISTORY:
• In May of 1801, while pondering
some of Newton’s experiments, Young
came up with a basic idea of Double
Slit Experiment to the demonstrate
the interference of the light wave.
5. INTERFERENCE
• Interference usually refers to the interaction of
waves that are coherent with each other,
either because they come from the same source
or because they have the same or nearly the
same frequency.
• If two waves have a definite phase relationship
then they are coherent.
• Otherwise, they are incoherent .
• For Interference:
• The sources must be Coherent.
• The sources should be Monochromatic.
• Principle of Superposition.
6. TYPES OF INTERFERENCE
• There are two types of interference:
1. Constructive
2. Destructive.
• Constructive interference occurs when the
wave amplitudes reinforce each other,
building a wave of even greater amplitude.
• Destructive interference occurs when the
wave amplitudes oppose each other,
resulting in waves of reduced amplitude.
7. Interference Of Light
• For constructive interference, path
difference between two waves is ml
i.e. path difference = ml
or path difference = 0, l, 2l, 3l, 4l, …..
where m = order = 0, +_1, +_2, +_3,
• If the path difference between two light
waves is (m+1/2)l , then the interference
between them will be destructive.
Path difference = (m+1/2)l
I.e. path difference= 1/2l, 3/2l, 5/2l ,
….
where m = order = 0, +_1, +_2, +_3,…..
8. Interference Of Light In Our Daily Life
• One of the best examples of
interference is demonstrated by the
light reflected from a film of oil floating
on water.
• Another example is the thin film of a
soap bubble, which reflects a spectrum
of beautiful colors when illuminated by
natural or artificial light sources. This
interactive tutorial explores how the
interference phenomenon of light
reflected by a soap bubble changes as a
function of film thickness.
9. INTRODUCTION
• Thomas Young
• (1773-1829)
• Thomas Young was the British
Physician.
• Thomas Young was the first to
describe
• properties and interference of
light.
• Young has been described as:
• "The Last Man Who Knew
Everything"
10. DOUBLE SLIT EXPERIMENT
INTRODUCTION:
It generally refers to an experiment in which light is allowed to diffract
through slits which produces fringes, or wave-like interference patterns on an
opposing screen. A similar experiment was performed by Claus Jonson at the
University of Tubingen where beams of electrons showed similar interference
patterns. The results of this experiment are often taken as evidence of the
“wave-particle duality” predicted by quantum physics.
Young’s Double Slit Interference is also called Young’s Double Slit
Interferometer and is the modern form of the Double Slit Interference.
11. DOUBLE SLIT EXPERIMENT
EXPLAINATION:
In this experiment, Young used a monochromatic(single wavelength) and coherent
source(light). And he used two boards, one with a single slit and the second with two slits
and a screen was used to observe the interference of light. He placed the source behind
the board with single slit. The second board was placed in front of the first one. The light
passed from the slits, obeying the law of superposition.
He observed some bright and dark fringes on the
screen this was due to the constructive and
destructive interference of the light wave. The bright
fringes were produced by the constructive
interference and the dark fringes were produced by
the destructive interference.
12. a single slit and another board with
two slits and superposition
constructive destructive interference
Brightness is directly prop to amplitude
Part diff =m lambda m={012345…}
Part dif 2m+1(lambda/2
14. YOUNG’S DOUBLE SLIT EXPERIMENT
EXPLANATION:
A screen having two narrow slits is illuminated by a
beam of Monochromatic light. The portion of the
wave front incident on the slits behaves as a source
of secondary wavelet. The secondary wavelet leaving
the slits are coherent. Superposition of these waves
result in a series of bright and dark fringe which are
observed on a second screen placed at some distance
parallel to the first screen.
16. DERIVATION OF YOUNG’S DOUBLE SLIT
EXPERIMENT
Path difference:
P= S2P2-S1P2
S2P2-S1P2=(S2B+PB)-(S1A+PA)
=[D+( x+ d/2 )]2-[D+(x+ d/2 )]2
S2P2-S1P2=2xd
S2P-S1P= 2xd
S2P+S1P
Now, Assuming 0 and P are very close such that,
S1P+S2P=d
Hence, Path difference= 2xd
D
Path difference= x d
D
Path difference= xd/D
17. DERIVATION OF YOUNG’S DOUBLE SLIT EXPERIMENT
LOCATION OF BRIGHT FRINGE:
P=mλ
P= Xmd = mλ
D
m=1,2,3,4,……
xm = mλD (m=0,Xo=0)
D (m=1,x1=xD/d)
FRINGE WIDTH:
β= xm-xm-1
=mλD –(m-1)λd = λd
d d d
β=λD
dΒ=λD/d
18. DERIVATION OF YOUNG’S DOUBLE SLIT EXPERIMENT
LOCATION OF DARK FRINGES:
P=(2m-1)λ/2
P=Xmd =(2m-I)λ/2
D
m=1,2,3,4,……
Xm=(2m-1)λd
2d
FRINGE WIDTH:
β=Xm-Xm-1
[(2m-1)λD/2d]-[[2(m-1)-1]λD/d]
Β=λD
d
19. PRINCIPLES
Since light has wave properties , it will
experience interference (the addition of
waves).
This interference is like that seen with water
waves .
Whether you get Constructive or destructive
interference depends on the wavelength on
light .
White light is made up of many colors .
These different colors have different
wavelengths.
20. APPLICATIONS
• So now that the mathematics behind the Double-slit Interference is known , what can they
tell us about light and how can they be used to help us better understand the world ?
• The Double-Slit experiment helped to establish the wave-particle duality theory of light ,
which we take today to be the best way to explain the behavior of light .
• We can determine the spectral order of any point on an interference pattern , provided we
know the angle and the wavelength or they can be calculated .
• Additionally , the wavelength or the angle can be determined if the other two necessary
variables are known or can be calculated .
• By determining the wavelength , other characteristics of the original light can be determined
. This principle will eventually lead to Diffraction Grating ,which can be used to determine
the characteristics of an element based on the color (wavelength) of the light it produces
when white light is shone through it .
21. USES OF
YOUNG’S
DOUBLE SLIT
EXPERIMENT
Young’s double slit experiment
provides a method for
measuring wavelength of the
light
This experiment gave the
wave model of light a great
deal of credibility
It is inconceivable that
particles of light could cancel
each other
23. NUMERICALS
Yellow sodium light of wavelength 589nm, emitted by a single source passes through
two narrow slits 1.00 mm apart. The interference pattern is observed on a screen
225cm away. How far apart are 2 adjacent bright fringes.
GIVEN DATA:
Wavelength of sodium light = λ = 589nm = 589X10-9m
Distance between the slits = d = 1.00X10-3m
Distance between the slits and screen = L = 225cm = 2.25m
TO FIND:
Distance between two adjacent bright fringes = Δy = ?
FORMULA:
ΔY = λL/d
SOLUTION:
Δy = (589X10-9mX2.25m)/1.0x10-3
Δy = 1.33X10-3m or 1.33mm
24. NUMERICALS
The distance between the slits in Young’s double slit experiment is 0.25cm. Interference fringes are formed on the
screen placed at the distance of 100cm from the slits .The distance of the third dark fringe from the central
bright fringe is 0.059cm. Find the wavelength of the incident light.
GIVEN DATA:
Distance between the slits = d = 0.25cm = 2.5X10-3m
Distance between the third fringe and the central bright fringe = y = 0.059cm = 5.9X10-4
Distance between the screen and slits = L = 100cm = 1m
For the third dark fringe m = 2
TO FIND:
Wavelength = λ = ?
FORMULA:
y = [m+1/2] λL/d
SOLUTION:
λ = yd/[m+1/2]L
λ = (5.9X10-4m X 2.5X10-3m)/(2+1/2)X1m
λ =5.90X10-7m = 590X10-9m = 590nm
