Since light is reflected from any surface that separates regions of different refractive indices, we can get interference effects when light falls on a thin film of transparent material with smooth surfaces. Light will be reflected from both the front and back surfaces and, so long as there are only two materials involved, one of the two surfaces will result in a 180° phase change in the reflected light. Light reflected from the two surfaces will interfere and the reflection will be strengthened or weakened depending on the phase change caused by travelling through the thin film twice. If the thin film has thickness L then the maximum reflection for light falling normally on the surface will be for wavelengths of the form 2L=(n+1/2)λm where λm is the wavelength measured in the thin film.
When white light falls on such a film different colours will be reflected by different amounts. This causes the colours seen in soap bubbles and in films of oil on water.
Because the reflection is colour dependent, the transmitted light will also be slightly coloured, in the complementary sense (if red is reflected then it will be missing from the transmitted light etc.). This effect is very weak because only a small fraction of the total light is reflected.
Coherent monochromatic light falling on a single slit of width w will produce a diffraction pattern in the far field (Fraunhoffer diffraction). The amplitude of light reaching the far screen will be given by
E0sinc(kwsin(θ)/2)
where sinc(x) = sin(x)/x and k is the wavenumber of the light, k=2π/λ.
The pattern has a central maximum that is twice as wide as the rapidily diminishing, equally spaced, maxmima surrounding it. The positions of the minima are given by wsin(θ)=nλ for all integers n except 0.
Coherent monochromatic light falling on a set of equally spaced slits with finite width w will produce a pattern that exhibits features of both single-slit and double-slit diffraction. The overall pattern will have the form of the single slit pattern for that slit width w, while the light within the broad peaks will be split into finer striations having the separation apropriate to the slit spacing d.
The actual intensity of a two-slit diffraction pattern is given by I∝(1+cos(k×d×sinθ))2.