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Non-reflective coating
Good quality lenses in a camera reflect very little light and
appear dark or slightly purple. A thin coating of a fluoride salt
such as magnesium fluoride on the surface of the lens allows
the majority of the light falling on the lens to pass through. The
refractive index, n, of the coating is chosen such that
1 < n < nglass
Air: n=1
Coating: n=1.38
Glass: n=1.50
Air: n=1
The rays are both
reflected from a material
of high refractive index,
so they both go under a
phase change of π.
Coating: n=1.38
Glass: n=1.50
For cancellation of reflected light (destructive interference) :
•optical path difference =λ/2 (minimum thickness)
•optical path in fluoride = 2nd
•thus 2nd =λ/2
•and
d λ
4n
Wedge Fringes
When a thin edge of air is illuminated with monochromatic light, a
series of light and dark fringes is observed due to the varying
optical path difference along the wedge.
Consider two glass microscope slides of length, L, separated by
a diameter, D, at one end.
Division of amplitude takes place at the lower surface of the top
glass slide.
When viewed from above
the
optical path difference = 2t
There is a phase difference of π on reflection at A. Hence the
condition for a dark fringe (destructive interference) is:
2t = mλ assuming an air wedge.
For the next dark fringe t increases by λ/2
The spacing of fringes, Δx, is such that
λ
tanθ 
2 x
Giving
λ
Δx 
2tanθ
For a wedge of length L and
spacing D.
D
tanθ 
L
The fringe spacing is
given by
λL
Δx 
2D
where λ is the wavelength
of light in air.