Question

Why UV rays harmful for us???

Answer 1

Uv rays are highly harmful for us because the exposure to the UV rays is a major risk factor for the skin Cancer that can lead to death. Sunlight is a major source of UV rays but most of the UV rays and many more harmful rays get deflected back due the presence of ozone layer in the atmosphere. People who get a lot of UV rays have a chance of getting skin cancer.

Answer 2

UV rays cause a chemical reaction when they strike DNA. More specifically, the kind of UV that comes down to the Earth's surface from the Sun dimerizes thymine and cytosine, meaning that whenever you have two thymine or cytosine bases (the "letters" of the genetic code) next to each other, it forms a "bar" of chemical bond between them. This deforms the DNA and derails the enzymes that read it when they hit that section, or causes them to malfunction and misread the code. As a result, that part of DNA stops working correctly.

If this piece of DNA was important for the cell's survival, it may cause it to die. If it controls the cell's reproduction, safeguards may still kick in and kill the cell by activating it's self-destruct sequence (yes, cells have a self-destruct button on them just like the Starship Enterprise), but if they don't or are themselves damaged, it may go on to reproduce abnormally, and accumulate further mutations. Eventually, a tumor may develop. This is skin cancer.

At higher frequencies -- higher than those which can reach the surface of the Earth because the atmosphere absorbs them -- the UV rays start to gain the ability to ionize molecules and atoms (strip off electrons), like X-rays which follow UV as the next EMR band, though without the same penetrating power as the wavelength is longer. This makes the waves even more damaging to DNA and cells.
hope you understand