Our Skin Can "See" UV Rays

Duh!  Obviously our skin reacts to ("sees") ultraviolet light. 

What is interesting in that the skin uses the same light receptor as in our eyes, called rhodopsin.

But if UV is non-ionizing, how does it cause cancer?

UV is between visible light (non-ionizing) and x-rays (ionizing) in terms of energy.  It is also subcategorized based on energy into A, B and C.

UV-C is mostly trapped in the upper atmosphere courtesy of ozone (that's why we care about the ozone hole).  UV-A, closest in energy to visible light, is too weak to be much of a player.

But UV-B is "just right".  It has enough energy to break covalent bonds associated with thymine and cytosine (the pyrimidine bases) of DNA.  Normally, thymine bonds with adenine and cystosine bonds with guanine (adenine and guanine being the purine bases):

Those open bonds can form dimers (T-T or C-C bonding) which can be disruptive to cell function if not repaired.

Learn more here.

This link explains the symptoms of basal cell and squamous cell cancers, the least dangerous types of skin cancer.

Melanoma is much more dangerous.