Thin Film Interference & Diffraction PDF

**Thin film interference** The multicolored patterns on an oil or gasoline slick on a wet pavement, or in soap bubbles are common examples of thin film interference. ***Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film in...

**Thin film interference** The multicolored patterns on an oil or gasoline slick on a wet pavement, or in soap bubbles are common examples of thin film interference. ***Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light.*** When the thickness of the film is an odd multiple of one quarter-wavelength of the light on it, the reflected waves from both surfaces interfere to cancel each other (assuming the refractive index of the thin film has a value between the refractive indices of the materials above and below it). Since the wave cannot be reflected, it is completely transmitted instead. When the thickness is a multiple of a half-wavelength of the light, the two reflected waves reinforce each other, increasing the reflection and reducing the transmission. Thus when white light, which consists of a range of wavelengths, is incident on the film, certain wavelengths (colors) are intensified while others are attenuated. Thin-film interference explains the multiple colors seen in light reflected from soap bubbles and oil films on water. It is also the mechanism behind the action of antireflection coatings used on glasses and camera lenses. If the thickness of the film is much larger than the coherence length of the incident light, then the interference pattern will be washed out due to the line width of the light source. Figure 7.4 (a) and (b) shows thin film interference of light in oil and glasses. Reflected wave is inverted Incident wave **Refracted waves are not inverted** **7.2 Diffraction of Light** When a beam of light passes through a narrow slit, it spreads out to a certain extent into the region of the geometrical shadow. This effect is one of the simplest examples of diffraction, i.e, the failure of the light to travel in straight lines. This phenomenon can be satisfactorily explained only by assuming a wave character of light as shown in figure 7.5. It is diffraction that gives the slits in Young\'s experiment the character of point sources. This depends on the slit width and wavelength of the light. To have appreciable diffraction, the slit width must be on the same order or less than the wavelength in figure 7.6. When the slit width is much smaller than the wavelength, spherical waves spread out from the slit as through they were from a point source. Notice in the figure the bending or diffraction around the edge of the slit. If the slit width is greater than the wavelength, little diffraction occurs.