Slits

A slit is a narrow, elongated opening or gap that restricts the passage of light, other waves, or particles. Slits appear in optical instruments, experimental apparatus, and engineered materials where controlled transmission is required. They can be simple mechanical gaps, components of optical systems, or features produced by fabrication processes such as lithography or laser drilling.

In wave optics, slits are central to diffraction and interference phenomena. The behavior depends on the wavelength λ relative to the slit width a. For a single slit of width a, monochromatic light produces a diffraction pattern with destructive minima at a sin θ = mλ (m = ±1, ±2, …). The central bright region is flanked by progressively weaker rings, and the approximate width of the central maximum on a screen a distance L away is about 2Lλ/a for small angles.

A pair of slits separated by distance d generates an interference pattern with bright fringes at d sin θ = mλ. The intensity is modulated by the diffraction envelope created by the finite slit width, so some fringes may be dimmer. When many slits are used, as in diffraction gratings, the maxima become sharper and more evenly spaced, following approximately mλ = d sin θ for the grating spacing d.

Applications include defining entrance apertures in spectrographs, improving spatial resolution in optical systems, and illustrating fundamental wave phenomena in demonstrations and experiments. Slits are also manufactured in metallic, glass, or semiconductor substrates using precision machining, laser drilling, or photolithography, enabling micro- to nanoscale control of light and particles for research and technology.