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The Shadow Blister Phenomenon
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The shadow blister effect, depicting the distortion of shadows when two objects overlap, is an optical phenomenon observable in sunlight without requiring specialized lab equipment.
Despite its seemingly straightforward nature, this effect challenges explanation through ray theory and the Fresnel diffraction equation in certain regions. Conversely, the shadow blister effect exhibits both linear and nonlinear behavior corresponding to the steady variation of the transverse distance between the two unplanar straightedges along the optical axis.
Similar to the conventional outdoor shadow blister experiment conducted on a sunny day, the experimental results of the shadow blister effect using a Galilean beam expander with a circular and a rectangular barrier and lasers with different wavelengths as the light source reveal the deforming of the shadows but more accurate with more details.
This article explores the shadow blister effect alongside the diffraction of a straight edge, revealing fundamental aspects of the diffraction phenomenon. The experimental study introduces a diffraction model adept at elucidating the shadow blister effect. This model relies on an inhomogeneous fractal space, potentially generated by objects near their surfaces, including the edges of barriers.
Considering the experimental results demonstrates that we need to be more precise and study the structure of the diffraction pattern as the general form of the shadow blister for the transverse distance between the barriers when it is smaller or equal to zero, and when it is larger than zero but less than the amount of the transition point, and when it is larger than the amount of the transition point but less than a millimeter, and finally when it is larger than a millimeter.
Upon thorough examination of the cross sections and diffraction patterns resulting from two parallel straight edges, which may lead to the formation of a shadow blister, three distinct boundaries become evident, corresponding to four conditions based on the transverse distance between the two edges along the X-axis. In cases where the transverse distance is sufficiently large, “Fresnel Diffraction” through a straightedge proves to be a valid approach for evaluating intensity at any arbitrary point on the observation plane and determining the position of the fringes. Moreover, the diffraction pattern maintains a fixed position due to the fixed position of the primary barrier.
However, in the second stage, as the transverse distance reduces to approximately a millimeter, the validity of the “Fresnel Integral” diminishes. In this scenario, fringe displacement experiences non-linear behavior with positive acceleration, corresponding to the constant velocity of the secondary barrier, until the transverse distance reaches a smaller value, considered a transition point. Subsequently, in the third stage, the displacement of fringes, corresponding to the steady speed of the secondary barrier, undergoes non-linear behavior with negative acceleration until the slit width reaches zero, and the “Fresnel Integral” remains invalid. In the final stage, as the transverse distance approaches zero, and simultaneously when the secondary barrier overlaps the primary barrier, the “Fresnel Integral” becomes valid once again. Moreover, the displacement of fringes, corresponding to the steady speed of the secondary barrier in this region, is linear with a constant speed and zero acceleration. Notably, complexity arises when the transverse distance is very small. In this condition, the Fourier transform is valid only if we consider a complex refractive index, suggesting an inhomogeneous fractal space with a variable refractive index near the surface of the obstacles. This variable refractive index causes a time delay in the temporal domain, leading to a particular dispersion region that underlies the diffraction phenomenon.