When light travels from one transparent material into another, air into water, for instance, part of it reflects back off the surface while the rest continues into the new material, but bent, refracted, at a different angle than it was originally travelling. This bending isn't random or approximate: Snell's law relates the angle of refraction directly and precisely to the ratio between the two materials' refractive indices, a property tied directly to how fast light actually travels through each material.
Refraction happens because light travels at different speeds in different materials
Light travels fastest through a vacuum, and more slowly through denser transparent materials like water or glass, with the exact slowdown for a given material captured by its refractive index. When light crosses the boundary between two materials with different refractive indices at an angle, one side of the light wave reaches the new, slower material and begins slowing down before the rest of the wave does, and this uneven slowdown across the wave's width is exactly what causes the light's direction of travel to bend at the boundary, rather than continuing on in a perfectly straight line.
Snell's law turns that bending into a precise, calculable prediction
Snell's law expresses this relationship precisely: the ratio of the sines of the angle of incidence and the angle of refraction equals the ratio of the two materials' refractive indices. Given a known angle at which light strikes a boundary and the refractive indices of the two materials involved, Snell's law lets a physicist calculate the exact angle at which the light will continue travelling into the second material, which is exactly the calculation behind everyday visual effects like a straw appearing to bend at the water's surface, or a lens focusing light to form a sharp image.
What we're still unsure about
Snell's law itself, and the underlying physical explanation connecting refraction to a material's refractive index, are precisely defined, extremely well established classical optics, confirmed across an enormous range of practical applications from eyeglasses to fibre-optic cables. What's more genuinely an ongoing area of applied optical engineering is designing materials and structures with more exotic, finely tuned refractive properties, including metamaterials engineered to bend light in ways ordinary natural materials can't achieve, for uses like advanced imaging or cloaking-style optical effects — researchers continue actively developing these engineered materials, without every theoretically possible refractive behaviour yet being achievable in a practical, manufacturable material.
This sits inside Reflection & Refraction (Snell's Law), one of eight topics in Waves & Optics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.