Refraction

Refraction is a fundamental wave phenomenon in physics in which a wave changes direction when passing from one transmission medium to another. This change of direction results from a change in wave speed as the wavefront crosses the interface between the materials. Although most commonly associated with light, refraction affects many types of waves, including sound waves and water waves. Its effects underpin the operation of optical instruments such as lenses and prisms, as well as the focusing mechanism of the human eye.

Principles of Refraction

Refraction occurs because waves generally travel at different speeds in different media. When a wave encounters the boundary between two materials at an angle, one side of the wavefront enters the second medium before the other side. If the second medium is one in which the wave travels more slowly, the leading part of the wavefront slows first, causing the entire wavefront to pivot towards the normal. Conversely, when the wave enters a medium in which it travels faster, the wavefront pivots away from the normal.

Wave Behaviour at the Boundary

Two interconnected phenomena underpin refraction:

• Slowing of light in materials: Light propagates more slowly in media such as water, glass or air than in a vacuum. This reduction in speed does not arise from absorption but from interactions between the electromagnetic wave and the electrons in the material. Oscillating electrons emit secondary electromagnetic waves, which combine with the incident light to create a resultant wave with reduced phase velocity.
• Change of direction: When light enters a medium at an angle, refraction alters the propagation direction. Once the light is fully within the new medium, it again travels in a straight line, though at a modified phase velocity. If the wavefront is perpendicular to the boundary, no change in direction occurs even though the speed changes.

Changes in wavelength accompany this process. The frequency remains constant across the boundary, but the wavelength is proportional to the wave speed, producing a shorter wavelength in a slower medium and a longer wavelength in a faster medium. This geometric change, together with boundary direction, yields the observed bending behaviour.

Dispersion of Light

Refraction is wavelength-dependent because the refractive index of a material varies with frequency. This phenomenon, known as dispersion, causes different colours of light to refract at different angles. As a result:

• Prisms spread white light into a spectrum of constituent colours.
• Rainbows form when sunlight is refracted and reflected within water droplets.

Shorter wavelengths (blue and violet light) generally refract more strongly than longer wavelengths (red light). Materials such as glass and water display this property prominently.

Refraction at Water Surfaces

A familiar example of refraction occurs where air meets a body of water. With the refractive index of air close to 1 and that of water around 1.33, light rays bend as they exit water into air. This causes objects partially submerged—such as a pencil—to appear bent or displaced. To an observer above the water, objects appear shallower than their actual depth; this phenomenon is known as apparent depth.
This effect has practical consequences. Spearfishers must aim below the visual position of a fish to compensate for refraction. Conversely, an object above the water surface appears higher when viewed from underwater, a phenomenon used advantageously by species such as the archerfish.
The ratio of apparent depth to real depth simplifies to the refractive index ratio when the angles involved are small. At high angles of incidence, reflection dominates, reducing visibility.

Atmospheric Refraction

The refractive index of air varies with density, which in turn depends on temperature and pressure. Air at higher altitudes has lower density and therefore lower refractive index than air near the surface. As light travels through layers of air with varying refractive indices, the rays gradually bend, often curving downward toward Earth.
Atmospheric refraction is responsible for:

• The apparent shift in the position of stars near the horizon.
• The Sun becoming visible slightly before its geometric rise and remaining visible after its geometric set.
• Mirage effects and heat haze, where temperature gradients distort the refractive index and cause shimmering or displacement of distant objects.

Such optical distortions pose challenges for high-magnification photography and astronomical observations, often limiting clarity in conditions where temperature gradients are strong.

Broader Applications

Refraction is central to many optical technologies. Lenses use refraction to focus or diverge light, forming the basis of cameras, microscopes and telescopes. Prisms exploit refractive differences to bend or separate light. The human eye also relies on the cornea and lens to refract incoming light towards the retina.
In acoustics, sound waves refract when travelling through layers of air with temperature or density differences, influencing how sound carries in outdoor environments. Water waves exhibit refraction when moving from deep to shallow regions, altering their propagation direction and behaviour near coastlines.