Shine a flashlight through a narrow slit, and the beam doesn't stay a thin stripe; it spreads out sideways. Light has a habit of bending around obstacles, and because of that, the smallest spot you can get with an ordinary lens is no smaller than half a wavelength. That limit seemed unbreakable until someone came up with the idea of sending light through a needle's eye while keeping it narrow.
Those surface waves catch the transmitted light and bend it toward the center, like two hands squeezing a stream of water. As a result, behind the film a light line forms that doesn't spread but actually narrows. At its tightest point, it measures 0.286 of a wavelength across, even though the limit for ordinary lenses is 0.5.
Earlier attempts hit a wall: the compressed light stayed glued to the surface and couldn't travel away. Here, calculations show that the energy flow points away from the film, meaning the focused light can propagate over a distance. It's as if a jet of water, after passing through a needle's eye, didn't spray everywhere but flew on as a narrow beam.
This approach could prove useful anywhere you need to work with very small details: in microchip manufacturing, data storage, or observing living cells. Light that has learned to pass through a needle's eye and stay narrow opens the door to instruments that can see things smaller than a wavelength.
Diffraction was first described by Leonardo da Vinci in the 15th century, and a precise description was given by Francesco Grimaldi in the 17th century. The slit width in the simulated lens (80 nm) is almost 8 times smaller than the wavelength of light (633 nm).