First, UV light also casts a shadow. So if there is a small piece of dirt on the dish, viruses under that dirt are protected from the light. Two dishes on top o

First, UV light also casts a shadow. So if there is a small piece of dirt on the dish, viruses under that dirt are protected from the light. Two dishes on top of each other, the viruses on the lower one are protected. Also, the lower part of the dish, and the table under the dish. Ultimately, you could only use the UV light to sterilize an empty table, assuming you cleaned it up first. But yes, this is how UV light is sometimes actually used. Shining on an empty table in a lab, for one hour. Even then, you don't take it as a 100% guarantee that there are no viruses left. Second, UV light hurts humans, too. Ignore the safety and remain in the room when the UV light is turned on, you get cancer. In the lab, people are careful. In an average household, it would be a question of time before someone gets exposed to the UV light. If you are careful, you can use the UV light on your room, or kitchen, and it is "better than nothing". (Not sure if you even need to cover your windows, because I think glass already reduces the UV part of light, plus the intensity is reduced by square of distance so you probably wouldn't hurt random people on the street.) But it takes a lot of time, and it only reduces the number of viruses by some fraction. Considering that COVID-19 is usually spread by air, not by touching surfaces, opening your windows probably achieves much more than using the UV light in your room or kitchen. For some other viruses, the numbers could be different. Also, the dishwasher already kills a lot of viruses by using higher temperature than when you wash dishes by hand.