Refraction of light is the change in direction and propagation behaviour that occurs when light passes between materials with different refractive indices, particularly when the light enters the boundary at an angle. The underlying reason is that the phase velocity of light differs between materials. In vacuum, electromagnetic radiation travels at its maximum speed, conventionally represented by the constant c, while in a transparent material its phase velocity is lower and depends on the material's optical properties and wavelength.
When a wavefront crosses an interface obliquely, one part of the wavefront can enter the new medium before another part. Because different portions then propagate at different speeds, the direction of the overall wavefront changes. The resulting change in direction is refraction. Refraction is responsible for the focusing action of lenses, the apparent bending of objects partly immersed in water, atmospheric optical effects, and many other phenomena. It is distinct from reflection, in which light is redirected at an interface without propagating into the second medium in the same way.
The quantitative relationship governing refraction at a simple boundary is Snell's law. It states that the refractive index of the first medium multiplied by the sine of the angle of incidence equals the refractive index of the second medium multiplied by the sine of the angle of refraction. The angles are measured from the normal to the interface rather than from the surface itself.
When light travels from a medium with a lower refractive index into one with a higher refractive index, the refracted ray bends towards the normal. When it travels from higher refractive index to lower refractive index, it bends away from the normal. If light enters the interface perpendicular to the surface, its direction does not change even though its speed and wavelength can change. The frequency remains determined by the source and does not change simply because the light crosses the boundary, while the wavelength changes because wavelength is related to propagation speed and frequency.
Refractive index is a dimensionless measure that describes how the phase velocity of light in a material compares with its velocity in vacuum. It is commonly defined as the speed of light in vacuum divided by the phase velocity in the material. Refractive index is generally wavelength dependent, a property known as dispersion. Consequently, different wavelengths of visible light can refract by different amounts when passing through the same material.
This is why a prism can separate white light into a spectrum. The different colours have different refractive indices and therefore follow different paths through the prism. Dispersion is also responsible for chromatic aberration in lenses because different wavelengths do not necessarily focus at exactly the same location. Optical materials are selected and engineered partly according to their refractive index and dispersion characteristics, which strongly influence lens design, transmission, reflection, and imaging performance.
Refraction can be understood through the wave nature of light as well as through ray geometry. In the wave description, the boundary changes the phase progression of the wave because propagation speed differs between the media. The wavefront rotates so that the direction of propagation changes while the frequency remains continuous across the boundary. In a simplified ray description, the light ray bends according to Snell's law.
The wave approach provides a deeper explanation for why refraction occurs and connects the phenomenon with interference, diffraction, phase velocity, and optical path length. Fermat's principle provides another useful perspective by stating, in simplified terms, that light follows a path for which the travel time is stationary with respect to small variations of the path. The principle can be used to derive the laws of reflection and refraction and illustrates that optical paths are determined by the properties of the media through which the light travels.
Refraction is essential to the operation of lenses. A lens has curved surfaces separating a transparent material from its surroundings. Each surface causes refraction, and the combined effects of the surfaces can cause light to converge or diverge. A convex lens in air typically converges light because the refractive index of the lens material is greater than that of air and the surfaces are curved appropriately. A concave lens normally diverges light under the same conditions.
The strength of a lens depends on its refractive index and surface curvatures as well as its thickness and surrounding medium. If the surrounding medium changes, the optical effect can change because the refractive index contrast between the lens and its environment changes. This is why a lens designed for use in air does not necessarily have the same focal properties when submerged in water. Refraction is therefore the fundamental mechanism through which transparent curved optical components control light.
Everyday examples of refraction are numerous. A pencil or straw partly submerged in water can appear displaced or bent because rays from the underwater portion change direction as they cross the water air interface before reaching the observer. A swimming pool can appear shallower than its actual depth because the refracted rays reaching the eye are interpreted as coming from a different apparent location.
Atmospheric refraction can cause distant objects to appear displaced, and variations in air temperature and density can bend light over long paths. Mirage phenomena occur when atmospheric refractive index varies with height, causing light to follow curved paths through layers of air. Refraction also affects the apparent positions of astronomical objects near the horizon because Earth's atmosphere bends their light. These examples show that refraction can occur in ordinary transparent materials as well as in gases whose refractive index varies continuously through space.
Refraction is not limited to visible light. Electromagnetic waves across different frequency ranges can experience changes in propagation speed and direction when entering materials, although the relevant refractive properties vary strongly with frequency. In optical fibres, controlled refraction and total internal reflection allow light signals to travel long distances with low attenuation relative to many other transmission methods.
Fibre optics rely on a core and cladding with carefully selected refractive indices so that light remains guided within the core under suitable conditions. In atmospheric science, radio waves can also be refracted by variations in atmospheric density and ionisation. In medical imaging, acoustics, and other wave based technologies, analogous concepts of changing propagation speed and bending occur, although sound is a mechanical wave rather than electromagnetic radiation. The general principle is therefore broader than visible light, even though the word refraction is often introduced through optical examples.
Refraction must be distinguished from several related phenomena. Reflection sends light back into the original medium, whereas refraction concerns propagation into another medium with changed speed and potentially changed direction. Diffraction involves the spreading and interference of waves around apertures, edges, or structures and is not simply the bending caused by crossing a boundary between materials. Scattering redirects light through interactions with particles or microscopic irregularities and can occur in many directions.
Absorption removes optical energy from the propagating radiation by transferring it to the material or other processes. At certain boundaries, both reflection and refraction occur simultaneously, so light energy can be divided between reflected and transmitted components. Under appropriate conditions, light travelling from a higher refractive index medium towards a lower refractive index medium can undergo total internal reflection instead of ordinary transmission. Refraction is consequently one component of a broader set of interactions between electromagnetic radiation and matter.
