The difference in phase between two points on a wave, often measured after reflection or transmission through a medium.Phase shift is the change in the phase of a wave—the position of its cycle relative to a reference point—after it interacts with a medium, reflects off a surface, or travels a certain distance. It is measured in degrees or radians, where a full cycle is 360° or 2π radians. Phase shifts are fundamental to wave phenomena: they determine interference patterns, the operation of lenses and prisms, and the behavior of light in optical fibers. In physics, phase shifts are not just a curiosity; they encode information about the medium a wave has passed through, such as its thickness, refractive index, or even the presence of a gravitational field.
The concept of phase was formalized in the early 19th century by Augustin-Jean Fresnel and Thomas Young, who used phase differences to explain interference and diffraction. The phase shift on reflection was experimentally demonstrated by Fresnel in 1823.
When a wave passes through a material, its speed changes, altering the wavelength while the frequency stays constant. This causes the wave to emerge with a phase that differs from what it would have in vacuum. For example, light passing through a glass plate is delayed, and its phase shifts by an amount proportional to the plate's thickness and refractive index. Reflection can also introduce a phase shift: a wave reflecting off a denser medium (like light hitting glass from air) undergoes a 180° phase flip, while reflection off a less dense medium does not. In interferometry, two waves that have traveled different paths are combined, and their relative phase shift determines whether they add constructively (bright) or destructively (dark).
Phase shifts are exploited in interferometers for measuring tiny distances (e.g., gravitational wave detectors like LIGO), in optical coatings that control reflection, and in medical imaging techniques like phase-contrast microscopy, which visualizes transparent biological samples.
In the Michelson interferometer used by LIGO, a phase shift of just 10^-10 radians—a fraction of a proton's diameter—is enough to detect the ripple in spacetime caused by a passing gravitational wave.
| Δφ | variable | Phase shift — phase shift radian |
| λ | variable | Wavelength of light — wavelength metre |
| Δn | variable | Change in refractive index — refractive index change |
| L | variable | Path length through medium — length metre |
