Diffraction is defined as the presence of light in the geometrical shadow of the sharp edge of an object when light passes over the edge. However the fringe pattern which tapers off from the edge of the surface of an object is also considered as diffraction pattern. But from the foregoing experiments it is construed that the diffraction pattern on the solid surface is due to the density ripples formed in the fluid layer on the surface. These density ripples are caused by thermal vibrations on the surface owing to the ambient temperature. The fringe patterns on the surface of a acrylic sheet is held in front of a vertical slit illuminated by a Sodium vapour lamp and studied. The acrylic sheet is aligned in such a away that the light reflected from the sheet surface gets refracted in the fluid layer and focuses and illuminates the first two fringes of the pattern. Then the acrylic sheet was heated with a candle and videographed (video clip - Diff).
Under the influence of heat the fringe pattern gets disturbed and the light's focus shifts away from the surface. On withdrawal of heat, the fringe pattern gets reformed and the light focus gradually shifts to the first two fringes. The heat is transmitted to the fluid layer whereby the density of the layer gets reduced and the fringes reposition themselves continuously in consonance with density changes. Due to density reduction the ability of the fluid layer to refract also reduces whereby the beam moves away from the surface. On withdrawal of heat the density in the layer gets restored to original state gradually and fringes are reformed and the light beam is back to the first two fringes. This is a typical case of density variation leading to variation in the refractivity as also fringes changing their position in consonance with the density variation. As the material in the acrylic sheet and the compressed portions in the ripples of the fluid layer refract light, light is found in the geometrical shadow of the acrylic surface. Contrary to the conventional use of glass plate for diffraction experiment, acrylic sheet is used as the projections on the surface of the glass plate are found impeding the propagation of fringes when heat is applied.
Video Clip - Diff