Engineering Physics

14.4 Interference in thin films by reflection

When white light (sun light) falls on thin films such as soap bubbles or air films then beautiful colours will appear due to interference. Interference of light in thin films can be explained by allowing monochromatic light of wave length ‘λ’ to fall on a thin film of thickness ‘t’ as shown in Fig. 14.4. Let the refractive index of film material be ‘μ’

AB is a monochromatic light beam incident at ‘B’ on a uniform thick transparent film. A part of the beam get reflected along BC and the remaining light is refracted along BD. A part of the refracted ray gets reflected at ‘D’ along DE. Again, this ray is refracted at ‘E’ along EF and enters into air. The ray BC and EF are parallel and they superimpose ...