How does UV rays kill germs but grow plants?
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When we use ultraviolet to sterilise things, we use UV-C, which is the highest frequency/energy of ultraviolet. This is not the same as what we use in ultraviolet lamps for plants, which use UV-A and/or UV-B.
The lower energy of the light used for plants means they don’t do as much damage, as it is not ionising to most materials.
UV rays from the sun are harmful to human skin and can cause sunburn, skin cancer, and other skin damage. However, these same UV rays can also kill germs. This is because UV rays damage the DNA of bacteria, viruses, and other microbes. This damage can prevent the microbes from reproducing, and eventually leads to their death. While UV rays can kill harmful microbes, they also help plants to grow. This is because UV rays help plants to produce vitamin D, which is necessary for plant growth.
UV is not a specific “color” of light, it is a range of colors and has wavelength of about 100 to 400nm.
UV-C (~100-280nm) will not grow plants and will kill germs (and many other cells) by breaking the bonds holding together DNA. The good news is that this UV is very easy to block. The atmosphere easily absorbs almost all of the Sun’s UV-C. Fluorescent bulbs are probably the most common UV-C sources you’ll encounter. The glass of the tube would block most of the UV-C even if the phosphors didn’t capture a lot of it and convert it to visible light. Germicidal UV-C source must be designed with special glass (fused quartz) which is fairly transparent to the higher range of UV-C (the extremely shortwave UV-C is blocked by just a little air).
UV-B and UV-A are useful AND dangerous for plants and animals. The UV can be used to aid in the synthesis of necessary compounds, but it can also generate others that can damage DNA. You body can clean up some of this mess, but if too much occurs then the body may not be able to keep up. Humans produced melanin (the skin pigment that produces tans and makes brown and black people darker than white people). Melanin helps regulate the skins exposure to UV.
In short, the UV that is used to kill germs is different from the UV that is beneficial to life, and too much of something is frequently bad for you.
UV rays kill germs by damaging their DNA. This makes it difficult for the germs to reproduce, which eventually leads to their death. Plants, on the other hand, use UV light to produce food. The process of photosynthesis converts sunlight into chemical energy that plants can use to grow.
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