  The epidermis is the outermost layer of the skin, and it plays a crucial role
in protecting the body from external insults such as UV radiation and physical
trauma. The stratum corneum is the topmost layer of the epidermis, composed of
dead skin cells and characterized by low water content. This low water content
creates a gradient in the refractive index. The current work aims to elucidate
the impact of a significant gradient of water content and, consequently, the
variations of the refractive index of the skin on light propagation in tissues.
Using analytical models of light propagation in single-layer and two-layer
tissues, we predict light confinement in the stratum corneum layer. For
example, the light intensity in the stratum corneum layer is noticeably
(11-17%) higher than in the underlying tissue layer. This effect can be
attributed to the high refractive index of the stratum corneum caused by low
water content, compared with underlying tissues, and scattering in the stratum
corneum layer. The effect is the most prominent for smaller diffuse reflectance
of the underlying tissue. Furthermore, the effect is expected to be maximal if
the thickness of the stratum corneum layer is more than the reduced scattering
length. Therefore, in the visible range of the spectrum, the light confinement
phenomena should be more noticable in stratum corneum layers with a thickness
of at least 150um, which can be found in the glabrous skin of palms and soles
and thickened epidermis-like calluses and corns
