Non-reflective coating

1. 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

2. 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. Glass: n=1.50 Coating: n=1.38 • For cancellation of reflected light (destructive interference) : • optical path difference =λ/2 (minimum thickness) • optical path in fluoride = 2nd • thus 2nd =λ/2 • and

3. 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 lowersurface of the top glass slide. When viewed from above the optical path difference = 2t

4. 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

5. Giving For a wedge of length L and spacing D. The fringe spacing is given by where λ is the wavelength of light in air.