$$*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:^{[33]}
where the numerical aperture is defined as $\mathrm {NA} =n\sin \theta \;$ 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:^{[34]}

- $\delta =1.22{\frac {\lambda }{D}}\ ,$

where λ is the wavelength of the waves that are focused for imag

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:^{[34]}

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.

## Subwavelength

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 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 p

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). It is equal to the "regular" wavelength "reduced" by a factor of 2π (*ƛ* = *λ*/2π). It is usually encountered in quantum mechanics, where it is used in combination with the reduced Planck constant (symbol *ħ*, h-bar) and the angular frequency (symbol *ω*) or angular wavenumber (symbol *k*).

## See also

## References