Question

please help with both pages. thank you in advance.

1. Why is the wave model, not ray model, of light is accurate when discussing diffraction and interference? 2. Describe the effects of diffraction; the bending of light waves, as an aperture (opening) becomes smaller and approaches the wavelength of the light source 3. Describe the phase condition for two waves to create constructive interference. Write the mathematical expression for location of interference maximas 4. Describe the phase condition for two waves to create destructive interference. Write the mathematical expression for interference minimas

Answer 1

Ans 1.} Consider a source of light(S) emitting light in all directions. Let a screen be placed at some distance from it. Place an opaque obstacle in between S and screen.According to geometrical considerations, light should be there on the screen above A' and below B'. There is supposed to be complete darkness in between A' and B'. Experimental observations, however, indicate that some light is present immediately below A' and above B'. But ray picture of light, according to which light travels in a straight line, could not afford an explanation to this experimental observation.

geometrical shadow. s obstac E screen

Ans2} Diffraction is explained on the basis of Huygens wave theory of light. If the size of the aperture (or diffracting obstacle) is much larger than the wavelength of light the incident plane wavefront will pass through the slit without change in its shape and direction of motion as shown in figure(a) resulting in rectilinear propagation of light.

Lightd d >> (a) Rectilinear propagation Light Light Light ad (b) Diffraction propagation

However, if the width of the aperture is comparable to the wavelength of light, most of the incident wavefront will be obstructed and in accordance with Huygens wave theory a cylindrical (a spherical) wavefront depending on the aperture (slit or hole) will originate from it. Now, as the wavefront, after passing through the aperture light will flare out as shown in figure (b). This flaring out or spreading of light is the so called diffraction.

ative interference r two wares to create constructive inter the phase difference b/w 2 waves must be an intence multiple of 2 T ite. - 0= 2nT & where nzo, 1, 2, 3,...n i n= 0,1,2 ,3, .... soon. Yozo gap 42 = 2AD IN - NAD central Maxima ist Maxing and maxime nth Maxima. Arsyl for two waves to create destructive interference the phase difference b/w Qwares mest be the odd multiple of T ire. 0 = (2n-1) T in= 1, 2, 3, for mining of interference path difference, A=(n-1) a=dsino for very small o tano ? sino tamo ng - Y = (2n-1) AD n=1723, di of - ad for, n=1) 1st Mining I = 2D , n= 2 2 Mining in In ory and minima similarly