How does light refraction occur?
Refraction is a change in light's direction as it crosses between media with different optical properties. It occurs because light's propagation speed changes. A spoon in water looks bent because rays change direction at the water-air boundary.
Refractive index and angle
Refractive index compares light's speed in vacuum with its phase speed in a medium. Snell's law relates incidence and refraction angles to the two indices; angles are measured from the surface normal. Frequency remains unchanged at the boundary, although wavelength can change. Normally incident light may change speed without changing direction. Wavelength-dependent refractive index explains color separation by a prism.
Lenses and total internal reflection
Curved lens surfaces focus rays in eyeglasses and cameras. Light traveling from a higher to a lower refractive index can undergo total internal reflection above a critical angle, an effect used in optical fibers. Refraction is distinct from mirror reflection. Real lenses also have chromatic and geometric aberrations, so the ideal thin-lens approximation does not describe every design accurately.