Shadow blister effect



Note: Some researchers have attempted to explain the shadow blister effect using ray theory. However, they overlooked the complexities of the phenomenon when the transverse distance between two objects produces shadows. Here, two solutions are presented. The first is based on ray theory, including some images and animations produced by computer programs that do not match the experimental data. The second interpretation is based on the results of experimental research.

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The shadow blister effect is a visual phenomenon in which a shadow bulges (or blisters) as it approaches another.

The effect takes place when two objects are at varying distances between a non-point light source and a background upon which their shadows are cast. As the objects move transversely such that their shadows approach each other, the one nearest the light source begins blocking light from reaching the inside of the other object's penumbra, thereby expanding its umbra. This expansion of the further object's umbra continues until the umbras of both objects meet.

This effect can be demonstrated and understood using ray theory.



The shadow blister effect is commonly misconceived to be an illusion caused by the combining of the two object's penumbras, aided by factors such as diffraction, nonlinear response, and the eye's inability to differentiate between varying contrasts.

The shadow blister effect ''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 straight edges along the optical axis. 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.''

http://www.ej-physics.org/index.php/ejphysics/article/view/304

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 straight edge 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.