We
wish to coat a flat slab of glass
|
Solution: Click For PDF Version The crucial concept to be used in answering this problem is that when light incident from a medium with refractive index
is reflected from the interface of a medium with refractive index
 ,
with
 ,
the reflected wave undergoes a phase change of
 .
Light
of wavelength
As
shown in the figure
The condition
for destructive interference is that the phase difference between
the two waves be
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with a transparent material
so that light of wavelength
620 nm (in vacuum)
incident normally is not reflected.
We have to find the minimum thickness that the
coating should have.
is first reflected from the surface of the coating whose index of
refraction is 1.25. As the wave is incident from air whose index of
refraction is 1.0, it undergoes a phase change of
.
is the thickness of the coating and the wavelength inside the
coating
.
Therefore, the minimum thickness of the transparent coating of
refractive index 1.25 that should be put on the glass surface so
that there in no reflected light at near normal incidence will be