![]() It takes place at the upper boundary of the film. On the other hand, a 180° phase shift will be introduced in the reflected wave. If we look at the refractive index of air it is 1(nair = 1) whereas the film will have an index larger than 1 (nfilm > 1). When we take a soap bubble, light waves travel through the air and hits the soap film. Path difference = 2μt Cos r Thin-Film Interference Soap Bubble Where t is the thickness of the thin film In triangle AGC, Sin r = GC / AC –––––– (ii)įG = 2t cos r In triangle ACD, Sin i = AD / AC –––––– (i) Derivation of Path DifferenceĬonsider light reflecting off the top and bottom interfaces of a thin filmįrom the geometry of the triangles AEB, AFB, ABC, and ACF, When light is traveling from rarer medium to denser medium, the phase shift is 180 degrees. When light is traveling from denser medium to rarer medium, the phase shift is zero. The interference can be constructive or destructive depending on the phase difference between the two reflected light waves, resulting in the increase or decrease in the brightness of the reflected light.ĭuring constructive interference, the light of a particular wavelength increases in intensity whereas in destructive interference it decreases in intensity. The phase difference ø is 2π by wavelength (lambda) times the path difference (x) Relation Between Path Difference and Phase Difference The phase angle is the part of one complete wave cycle measured as a fraction of 2π (360 degrees) i.e the phase difference from one wave peak to the next is 2π (360 degrees). Path difference is defined as the difference in actual distance traveled by the two waves. The phase difference is the difference in phase angle between two waves. In this case, the phase difference between the waves is 180 degrees. Thus, the effective path difference between transmitted rays is also 2µt cos r. In this case, there is no phase change due to reflection at B and C because in both cases light is traveling from denser to rarer medium. Hard Reflection: Reflects off a medium with a higher refraction index, as a result, a phase shift is observed of λ/2. Soft Reflection: Reflects off a medium with a lower refraction index, as a result, no phase shift is observed. The form of reflection undergo by the light waves at each boundary depends on the refraction indexes of the two mediums. We will look at the major conditions for interference and get a better idea of the concept. Monochromatic source results in interference patterns that appear as light or dark bands. A source could be a monochromatic or broadband source. ![]() Wavelength and angle of the incident light.Īn important consideration in determining thin-film interference is also by knowing the source of the light. Meanwhile, the type of interference that can take place is dependent upon these factors. Thus, there are two waves emerging from a thin film – one wave reflected off the top surface of the film and the other reflected off the bottom surface. When the second medium is a thin film, there are two reflections occurring close together at the top and bottom boundary surfaces of the thin film. During this, the light reaches the boundary between two media, and part of it gets reflected and some part gets transmitted. When light waves that reflect off the top and bottom surfaces interfere with one another we see different colored patterns. Thin-film interference is the phenomenon that is a result of lightwave being reflected off two surfaces that are at a distance comparable to its wavelength. We shall study more in-depth about this concept below. This observable effect is known as thin-film interference. For instance, you must have noticed some rainbow-colored reflection in a pool of water on the side of the road or in a bubble. Thin-film interference is a common occurrence that happens in our everyday lives.
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