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Non-reflecting coatings

You may have noticed that many good camera and projector lenses appear bluish or purplish; this is due to a coating designed to prevent unwanted reflections from the lens surface. The surface of the glass is coated with a material which is transparent and has a refractive index between those of air and glass; magnesium fluoride, MgF2, with a refractive index of 1.38, is often used.

The thickness (t) of the film is chosen to give destructive interference between the beam reflected from the surface of the coating and that reflected from the surface of the lens (Figure 1).


This is given by 2nt = l/2 for normal incidence or 2nt cos r = l//2 for light that does not meet the surface at right angles; a value of t of around 10-5 cm is typical.

The value of the wavelength chosen is around 530 nm, because this is the wavelength to which the eye is most sensitive. Removing light of this wavelength from the spectrum leaves mainly blue and red, thus giving the characteristic purplish colour.

It can be shown that the two reflected waves from the coating and the glass have the same intensity if the refractive index of the coating is equal to the square root of the refractive index of the glass. If the refractive index of the coating is greater than that of the glass then the reflection is increased!

© Keith Gibbs 2010