Post on 15-Apr-2017
TEACHING PRESENTATION
IN PHYSICS
Philippe MANUEL
English - MASTER 1 EFTIS2012 -203
The Double Slit Interference Experiment•Introduction and history
•Wave interference properties
•The principle of superposition
•Thomas Young Experiment details
•Conditions for constructive/desctructive interferences
•Geometric statements about the interferences
•Exercise : calculation of a light wavelength
The Double Slit Interference Experiment
Conducted by Thomas Young in 1803
Showed an interference pattern
A breakthrough of the light corpuscular theory of Isaac NEWTON
Measured the wavelength of the light
Two Waves Interfering : Water waves example
waves characteristics :
Antinode <=>
Minimal disturbance
Node <=>
Maximum disturbance
Principle of SuperpositionWhen two waves coincide, their displacements are added vectorially.
+ =
The two waves are in phase or have a phase difference of 2 or
This process is called reinforcement orconstructive interference
Principle of Superposition
+ =
The waves are out of phase or have a phase difference of or /2.
Annulment ( Flat wave ) or
This process is known as
destructive interference
Young’s experiment details
Light source S
1
S2
Double Slit Interference Experiment
Diffraction step 1
Light source S
1
S2
Double Slit Interference ExperimentDiffraction step 2
S1
S2
s1
s2
Destructive
Dark fringe
Bright fringeConstructive
Double Slit Interference Experiment
How can we determine the conditions of constructive or destructive interference
( bright fringe versus back fringe ) ?
Typical Question
Bright fringes occur when the difference in path Bright fringes occur when the difference in path pp
is an integral multiple of one wave length is an integral multiple of one wave length ..
pp11
pp22
pp33
pp44
Path differencep = 0, , 2, 3, …
Bright fringes: p = n, n =
0, 1, 2, . . .
Dark fringes occur when the difference in path Dark fringes occur when the difference in path p p
is an odd multiple of one-half of a wave length is an odd multiple of one-half of a wave length ..
pp11pp22
2
pp33
pp33
2p n
n n = = oddoddn n = =
1,3,5 …1,3,5 …
Dark fringes:
1, 3, 5, 7, . . .2
p n n
Géometric statementsAssume L>> d
is very small !
Ld*n
X=L*-> nd*X/L For n=0,1,2,3,…
d
L
X
S1
S2
Conditions required : L=n
Double Slit Interferences conditions
SummaryLocation of fringes ( x) :
x n L/d x L/d
Constructive (brights) n=0,1,2,3,…..
Destructive (darks) n=1/2, 3/2, 5/2,….. ( m +1/2) ( odd numbers)
Application of wave length measurement exercise
Monochromatic light from a single slit illuminates two narrow parallel slits.
The centres of the two slits are 0.800mm apart. An interference pattern forms on a screen 50.0cm away. The fringe separation on the screen is 0.304mm.
Find the wavelength of the light.
Exercise - Answers
m
LxddLx
7
7
1
44
1086.4
10864.4 1000.5
1004.31000.8
d = 0.800mm = 8.00 x 10-4mL = 50.0cm = 5.00 x 10-1mx = 0.304mm = 3.04 x 10-4m