# What it takes to wash a strainer: soap, water and some wave optics

When I stay over at a friend’s place whenever I come to Delhi, I try to help around the house. But more often than not, I just do the dishes – often a lot of dishes. One item I’ve always had trouble cleaning is the strainer, whether a small tea strainer or a large but fine sieve, because I can never tell if the multicoloured sheen I’m seeing on the wires is a patch of oil, liquid soap or something else. The fundamental problem is that these items are susceptible to the quirks of the wave of nature of light, as a result of which their surfaces display an effect called goniochromism, also known as iridescence.

At first (and over 12 years after high school), I suspected the wires on the sieve were acting as a diffraction grating. This is a structure that has a series of fine and closely spaced ridges on the surface. When a wave of light strikes this surface, the ridges scatter different parts of the wave in different directions. When these waves interact with each other on the other side, they interfere with each other constructively or destructively. A constructive interference produces a brighter band of colour; a destructive interference produces a darker band. How the wave becomes scattered is a function of its frequency: the lower the frequency (or redder the colour), the more the wave is bent around a grating.

As a result, white and continuous light appears to breakdown into its constituent colours when passed through a diffraction grating. But it must be noted that a useful diffraction grating used in a visible-light experiment has something like 4,000-6,000 ridges every centimetre. The width of each ridge has to be of comparable size to the wavelength of visible light because only then can it scatter that portion of light. On the other hand, the sieve I was holding appeared to have only 6-8 ridges every centimetre, so the structure itself couldn’t have been what was effecting the sheen.

Goniochromism, or iridescence, is caused when two transparent or semi-transparent films – like liquid soap atop water – reflect the incident light multiple times. In fact, this is one type of iridescence, called thin-film interference. Here, imagine a thin layer of soap on the surface of a thin layer of water, itself sitting on the surface of a vessel you’re cleaning. (With a strainer, the water-soap liquid forms meniscuses between the wires.) When white light strikes the soap layer, some of it is reflected our and some is transmitted. The transmitted portion than strikes the surface of the water layer: some of it is sent through while the rest is reflected back out.

When the light reflected by each of the two layers interact, their respective waves can interfere either constructively or destructively. Depending on the angle at which you’re viewing the vessel, bright and dark bands of light will be visible. Additionally, the thickness of the soap film also decides which frequencies are intensified and which become subdued in this process. The total effect is for you to see rainbow-esque pattern of undulating brightness on the vessel.

So herein lies the rub. Either effect, although the second more than the first, produces what effectively looks like an oily sheen on the strainer in my hand no matter how many times I scrub it with soap and run it under the water. And ultimately, I end up doing a very thorough job of it if there was no oil on the strainer to begin with – or a very bad one if there was oil on it but I’ve let it be assuming it’s soap residue. It’s a toss-up… so I think I’ll just follow my friend C.S.R.S’s words: “Just rub it a few times and leave it.”

Featured image credit: Lumix/pixabay.