The diagram shows light incident from above on a film of thickness d. Each of th
ID: 1445034 • Letter: T
Question
The diagram shows light incident from above on a film of thickness d. Each of the three materials in the figure can be chosen to be air, with index of refraction n=1.00, water, with index n=1.33, or glass, with index n=1.50. Under which of the following conditions will the reflected light be completely or nearly eliminated by interference?
(lambda2 refers to the wavelength of the light inside the thin film.)
(Note: In the case that d<<lambda2, you can assume the thickness is so small that the travel distance in material 2 has negligible affect on the phase of the wave. Treat this as if the two interfaces are almost right on top of each other.)
d<<lambda2, material 1 is air, material 2 is water, material 3 is glass.
d=lambda2/4, material 1 is air, 2 is water, 3 is glass.
d=lambda2/4, material 1 is glass, 2 is water, 3 is glass.
d=lambda2/2, material 1 is air, 2 is water, 3 is glass.
d<<lambda2, material 1 is air, 2 is glass, 3 is water.
True/False
1|2| 3Explanation / Answer
The material 1 is air, material 2 is water, and the third material is glass.
as the refractive index of the water is more than the air, the phase shift of the reflected ray from top surafce of the film has pi radians.
as the refractive index of the glass is more than the water, the reflected ray from the bottom surface of hte film has pi radians
Therefore, the two ray reflected from the top and bottom surfaces of the film are in same phase. therefore, interfernce takes place between these two reflected rays.
if the reflected rays are disappeared, then the interfernce is dustructive.
The condition for destructive interference is,
path diff = (m+0.5)lambda2
2t = (m+0.5)lambda2
for minimum thickness,
2t = (0+0.5)lambda2
t=0.5* 0.5*lambda2=0.25lambda2