“DOUBLE SLIT DIFFRACTION”

PRESENTED BY SARA DEPARTMENT OF PHYSICS 3RD SEMESTER (B.S)

2

DIFFRACTION:

The bending of wave around an obstacle is called diffraction.

APPLICATIONS: Because of diffraction we can hear sound outside the close room.

3

INTRODUCTION:

• The double-slit experiment is a nineteenth-century investigation into the properties of light.• This experiment is sometimes referred to as

Young's experiment.• The experiment belongs to a general class of

"double path" experiments, in which a wave is split into two separate waves that later combine back into a single wave.

4

DOUBLE SLIT DIFFRACTION:

Young began his experiment by sending waves of coherent light towards a barrier with two slits. The two slits are separated by a distance d. On a screen, a perpendicular distance L from the slits, a series of interference fringes were viewed. The formulas we will now develop will allow us to determine if a point P in the interference pattern a distance y from its center will fall into a bright zone or into a dark zone.

5

6

7

EXPLAINATION:

EPD represents extreme path difference, or the effective difference in the distances the light must travel to reach a given position on the screen from each of the slits.  For bright fringes, this path difference must be a multiple of a wavelength to insure constructive interference. When this overall path difference equals an odd-multiple of a half wavelength, then destructive interference is insured and a dark fringe is formed on the screen.

8

CONDITIONS:

•  Thomas Young's equation for double slit interference is

• For bright fringes (constructive interference) 

 • For dark fringes (destructive interference)  

whereL

y

d

EPD'

sin L

ytan

mEPD ........)3,2,1,0(m

2)12(

mEPD ........)3,2,1(m

9

EXAMPLE OF LASER LIGHT:

10

LASER DIFFRACTION COMPARED TO INTENSITY DIAGRAMS

• The pattern formed by the interference and diffraction of coherent light is distinctly different for a single and double slit. The single slit intensity envelope is shown by the dashed line and that of the double slit for a particular wavelength and slit width is shown by the solid line. The photographs of the single and double slit patterns produced by a helium-neon laser show the qualitative differences between the patterns produced

11

LASER DIFFRACTION COMPARED TO INTENSITY DIAGRAMS

• The number of bright maxima within the central maximum of the single-slit pattern is influenced by the width of the slit and the separation of the double slits.

12