Slit
Blocks rays outside the configured aperture gap.
Open in the canvas →In the real world
A slit is an aperture: two opaque jaws with a gap between them. Unlike a beam dump, whose job is to stop a beam entirely, a slit stops only part of one — it passes the light within its gap and absorbs everything outside it. That makes it a shaping and selecting element, though the light it rejects still has to be absorbed, and at high power the jaws face the same thermal problem a dump does.
Slits do two quite different jobs depending on where they sit. In a plane where the beam is spatially spread out, a slit trims the beam's cross-section — cutting off a tail, defining a sheet of light, or setting the illuminated strip in a line-scan system. In a plane where wavelengths have been spread out by a grating or prism, exactly the same component becomes a wavelength selector: it passes a band and rejects the rest. A monochromator is, in essence, a dispersing element with a slit at each end, and the slit width sets the spectral resolution directly.
There is a limit to how far this can be pushed. Narrowing a slit does not narrow the transmitted beam indefinitely, because diffraction sets in: the narrower the aperture, the more the light spreads after it.
Because a slit rejects most of the light reaching it, it is a lossy component by design, and in a spectrograph the trade-off is explicit: a narrower slit buys resolution at the cost of signal. Choosing the width is choosing where on that curve to sit.
The rejected light does not vanish. On a low-power source it simply warms the jaws; on a high-power one the jaws need the same treatment as a beam dump — an absorbing surface that can shed heat, and at high enough power, active cooling. A pair of thin blackened blades that works at milliwatts will not survive tens of watts.
In OpticalSetup
The slit is drawn as two absorbing jaws with a gap between them, and it is traced exactly that way: rays passing through the gap continue completely unchanged, and rays striking either jaw are absorbed and removed. Two controls set it — the gap, and the overall optic size that fixes how far the jaws extend.
The useful consequence is that a slit here is a genuine spatial filter. Send a wide beam at one and only the central portion survives, so you can define a beam width mid-path, clip the wings off a diverging beam, or take one branch of a fan and discard the rest. Place one after a grating or a prism and it becomes a wavelength selector, because the colours have been separated in space by then and the slit is choosing among positions.
The purple canvas knob adjusts the gap directly, which makes the selection easy to explore: widen it until the branch you want passes, then narrow it until only that branch does.
Diffraction is not modelled, and for a slit that is the significant omission: narrowing the gap here simply passes a narrower bundle of rays, whereas a real slit below about a millimetre starts spreading the light it transmits, and a very narrow one produces a broad diffraction pattern rather than a thin beam. Nothing in this element will ever show that reversal. Transmission through the gap is also perfect and edge effects are absent — no partial transmission at the jaw edges, no scattering off them, and no wavelength dependence. As with the beam dump, the absorbed light produces no heat and carries no damage threshold, so a sketch will happily throw arbitrary power at a pair of thin blades.