Refractive index governs the bending angle
When light travels from one medium to another with different refractive indices, it bends at the boundary. Snell's law states n1 sin(theta1) = n2 sin(theta2), where theta1 and theta2 are the angles from the normal (perpendicular) to the surface. The refractive index n describes how much slower light travels in that medium compared to vacuum (c/v). Water has n = 1.33, so light bends toward the normal when entering water from air. Diamond has n = 2.42, causing dramatic bending and the characteristic 'sparkle' as light is severely bent and internally reflected. At a critical angle, light traveling from a denser medium to a less dense one is completely reflected (total internal reflection), the basis of fiber optics: light trapped in an optical fiber bounces off the cylindrical surface and travels without loss to the far end.
Optics and everyday phenomena
A pencil submerged in water appears bent because light from the submerged part refracts as it exits the water, creating an illusion of an offset position. Mirages in deserts arise from a gradient in air temperature creating a continuous gradient in refractive index: light from the sky bends as it passes through hotter, less dense air near the ground, creating an inverted image that looks like a reflection in water. Rainbows form when sunlight enters water droplets, refracts inward, reflects off the back surface, and refracts outward again. Different wavelengths refract at slightly different angles (dispersion), separating sunlight into its spectrum colors. Understanding refraction is essential for designing telescopes, microscopes, cameras, and any optical instrument where light must be focused or imaged precisely.