Snell's window is a phenomenon in which a viewer underwater sees the surface through a cone of light. The cone of light is created by refraction as the light hits the water. When the light hits the water it bends outward because it is refracted off the medium which would be the water. The light must hit at a angle to refract and create the cone. If the light comes in at no angle the light will not bend. The largest angle you can create is a 90°angle if it is above it will only be paralleling the water or medium. Snell's window is possible because of Snell's Law of Refraction. Refraction is when light hits a material and goes through it, but when it goes through it the light bends. The bending light creates the cone of light you see when you witness Snell's Window. Snell's Window is a cool science related thing that most people wouldn't think science would have something to do behind the creation of Snell's window.
