refractivity

Refractivity is a property of a medium that describes how it affects the propagation of electromagnetic waves, by slowing and bending the waves as they travel. In optics this is usually discussed in terms of the refractive index n, defined by n = c/v, where c is the speed of light in vacuum and v is the phase velocity in the medium. Refractivity is often represented by N = (n − 1) × 10^6, a convenient unit in atmospheric and radio contexts because it scales with the medium’s density and with temperature.

Refractive index depends on wavelength, a phenomenon known as dispersion. As a result, refractivity varies across the spectrum: different frequencies experience different bending and speeds in the same medium. In air at standard conditions, n is about 1.00027, corresponding to refractivity on the order of a few hundred N units; exact values depend on temperature, pressure, and humidity.

In the atmosphere, spatial gradients in refractivity cause bending of light and radio waves, leading to effects such as atmospheric refraction, mirages, and the apparent displacement of celestial objects. These gradients are also exploited in atmospheric sensing and radio science, for example in radar propagation models and radio occultation techniques. Refractivity is a general concept applicable to any medium, including glass, water, and optical fibers, where higher refractivity means stronger bending of light at interfaces and within the material. Measurements are performed by refractometry and analyzed through Snell’s law and ray tracing.