To prevent reflection from a camera lens, a thin film of magnesium is coated. In order that the light of wavelength 5000 Å not be reflected the minimum thickness of the film should be (µ for magnesium = 1.25 and µ for glass = 1.5) –
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The condition for destructive interference (minimum reflection) in a thin film, when light is incident normally and there is a phase change at both interfaces (air-film and film-glass, as 
), is given by 
, where n is the refractive index of the film, 
is the thickness of the film, m is an integer ( 
), and 
is the wavelength of light in air.
For the minimum thickness, m is taken as 0 .
The given values are 
and 
.
Substituting these values into the equation for destructive interference with 
, it is found that 
.
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