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According to this website, "UVA rays have the longest wavelengths, followed by UVB, and UVC rays which have the shortest wavelengths. While UVA and UVB rays are transmitted through the atmosphere, all UVC and some UVB rays are absorbed by the Earth’s ozone layer. So, most of the UV rays you come in contact with are UVA with a small amount of UVB."

But shorter wavelengths have more energy/penetration power, so why doesn't more UVC radiation reach the earth?

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Well, UV-C surely carries more energy than UV-A and UV-B, but the reason as to why UV-C radiation doesn't reach Earth is because of the presence of $O_2$ and $O_3$ in the stratosphere. UV-C radiation carries enough energy to excite an electron present in the oxygen molecule to shift to a higher energy orbital in the same oxygen molecule. Almost all energy from UV-C radiation is used up.

Now, this excited $O_2$ decomposes to form two free radicals of oxygen $[O]$ which further reacts with other oxygen molecules found in the atmosphere to create $O_3$ (Ozone) molecules.

In a way, the energy from UV-C is used up in creating $O_3$ molecules, therefore, there is no radiation reaching us. UV-A radiation, however, reaches us because $O_3$ is transparent to UV-A radiation, that is, UV-A radiation doesn't provide enough energy (wavelength is inversely proportional to energy) to excite the electrons present in $O_3$, so most of UV-A radiation reaches the Earth but certain UV-B radiations have the energy to excite electrons in $O_3$, therefore, not much of UV-B radiations reach the surface of the Earth.

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  • $\begingroup$ Thank you so much! :-) $\endgroup$ – laksheya Dec 10 '19 at 2:24

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