Minor Project

Minor ProjectA TABLE TOP EXPERIMENT TO MAKE FLOWING SOAP FILMS the simplest thin film

Presented by Surbhi Verma (2k11/EP/072) Under the guidance of Prof. R. K. Sinha

Project GoalsTo design and construct a novel, compact and simplified apparatus to make sustainable soap filmsTo understand interference in thin films with soap film as a thin filmTo understand the formation of black and colored bands in a soap film

Why Soap Films??

A soap film is a micrometer thick sheet of water covered on either side by surfactant molecules. A soap film is a thin film with refractive index almost equal to that of water and surrounded by air on both sides.It thus provides a compact, easy and inexpensive way to study interference and demonstrate experiments.

Mathematical Analysis Optical Path Difference, OPD = n2(AB + BC) n1(AD) Where, AB = BC = d / cos2 AD = 2d tan2 sin1 Using Snells law, n1 sin1 = n2 sin2 OPD = n2 ( 2d / cos2 ) - 2d tan2 n2 sin2OPD = 2 n2 d cos2 This is known as cosine law

Adding the extra phase change of (from denser to rarer medium) to the above path difference, conditions for constructive and destructive interference of reflected light are:

Constructive interference,

Destructive interference,

THE APPARATUS

Designing the Apparatus

Creating Soap Film

Relation between Intensity, film thickness and wavelength for normal incidence

Explanation of Black Band Light rays reflected from the outer surface with a phase change of 180 degrees will recombine with waves that have been reflected off of inner surface with no phase change.When the film becomes so thin that its thickness causes an insignificant phase shift, the result is complete destructive interference of the two light waves.

EXPECTED INTENSITY PLOTS

Intensity as a function of film thickness for red (650nm), green (550nm), blue (450nm) and violet (400 nm)

Intensity v/s film thickness (ranging from 0-200 nm)

Intensity v/s wavelength at low thickness values

Intensity v/s wavelength at greater thickness

Applications

Dichroic Filtersuse the principle ofthin-film interference, and produce colors in the same way as soap filmsThey are very accurate color filter used to selectively pass light of a small range of colors while reflecting other colorsalternating layers ofoptical coatingswith differentrefractive indexesare built up upon aglasssubstrateThe interfaces between the layers of different refractive index produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths

Anti-reflection coatingoptical coatingapplied to the surface oflensesand other optical devices to reducereflectionLayer thicknesses are chosen to produce destructive interferencein the beams reflected from the interfaces, and constructive interference in the corresponding transmitted beamsThis improves the efficiency of the system since lesslightis lost