The function "S" has zeros where "u" is a non-zero integer, where are at "x" values at a separation proportion to wavelength. Diffraction is the fundamental limitation on the resolving power of optical instruments, such as telescopes (including radiotelescopes) and microscopes. For a circular aperture, the diffraction-limited image spot is known as an Airy disk; the distance "x" in the single-slit diffraction formula is replaced by radial distance "r" and the sine is replaced by 2"J"1, where "J"1 is a first order Bessel function. The resolvable "spatial" size of objects viewed through a microscope is limited according to the Rayleigh criterion, the radius to the first null of the Airy disk, to a size proportional to the wavelength of the light used, and depending on the numerical aperture:  where the numerical aperture is defined as formula_15 for θ being the half-angle of the cone of rays accepted by the microscope objective. The "angular" size of the central bright portion (radius to first null of the Airy disk) of the image diffracted by a circular aperture, a measure most commonly used for telescopes and cameras, is:  where λ is the wavelength of the waves that are focused for imaging, "D" the entrance pupil diameter of the imaging system, in the same units, and the angular resolution δ is in radians. As with other diffraction patterns, the pattern scales in proportion to wavelength, so shorter wavelengths can lead to higher resolution. The term "subwavelength" is used to describe an object having one or more dimensions smaller than the length of the wave with which the object interacts. For example, the term "subwavelength-diameter optical fibre" means an optical fibre whose diameter is less than the wavelength of light propagating through it. A subwavelength particle is a particle smaller than the wavelength of light with which it interacts (see Rayleigh scattering). Subwavelength apertures are holes smaller than the wavelength of light propagating through them. Such structures have applications in extraordinary optical transmission, and zero-mode waveguides, among other areas of photonics. "Subwavelength" may also refer to a phenomenon involving subwavelength objects; for example, subwavelength imaging. A quantity related to the wavelength is the angular wavelength (also known as reduced wavelength), usually symbolized by "ƛ" (lambda-bar).