Chapter 35&36 Interference and the Wave Nature of Light

1. Light as a Wave 2. THE PRINCIPLE OF LINEAR

SUPERPOSITION3. Young's Double-Slit Experiment4. Diffraction

What is physics?

Light as a Wave

Huygens' principle: All points on a wavefront

serve as point sources of spherical secondary wavelets. After a time t, the new position of the wavefront will be that of a surface tangent to these secondary wavelets.

                                                                                                       

       

Constructive Interference

Condition: , m=0, 1, 2, 3, …. 2 1- l l m

Destructive Interference

Condition: , m=0, 1, 2, 3, ….2 11- ( )2

l l m

Young's Double-Slit Experiment

                                                                                                                                                                               

     

Bright fringes:

Dark fringes: 1sin ( )2

l d m

sinl d m

Where m=1, 2, 3, ∙∙∙

Example 1  Young’s Double-Slit Experiment Red light (λ=664 nm in vacuum) is used in Young’s experiment

with the slits separated by a distance d=1.20×10–4 m. The screen in Figure is located at a distance of L=2.75 m from the slits. Find the distance y on the screen between the central bright fringe and the third-order bright fringe.

Interference from Thin Films

         

                   

Condition for destructive interference is:

Diffraction

The diffraction is the bending of waves around obstacles or the edges of an opening

Diffraction determined by the ratio λ/W

Smaller λ /W, less diffraction Larger λ /W, more diffraction

Conditions for dark fringes

Conditions for dark fringes in single-slit diffraction:

sin where 1, 2,3,...w m m

                                     

                  

Example   Single-Slit Diffraction

Light passes through a slit and shines on a flat screen that is located L=0.40 m away (see Figure). The width of the slit is W=4.0×10–6 m. The distance between the middle of the central bright fringe and the first dark fringe is y. Determine the width 2y of the central bright fringe when the wavelength of the light in a vacuum is (a) λ=690 nm (red) and (b) λ=410 nm (violet).