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