The Ozone Layer is..............

a deep layer in the  stratosphere, encircling  the Earth, that has   large amounts of ozone  in it. The layer shields  the entire Earth from much of the harmful  ultraviolet radiation that comes from the sun. 

Interestingly, it is also this ultraviolet radiation  that forms the ozone in the first place. Ozone is a  special form of oxygen, made up of three oxygen  atoms rather than the usual two oxygen atoms. It  usually forms when some type of radiation or  electrical discharge separates the two atoms in an  oxygen molecule , which can then individually  recombine with other oxygen molecules to form ozone .The photo chemical mechanisms that give rise to the ozone layer were discovered by the British physicist Sydney Chapman in 1930. Ozone in the Earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms , splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken oxygen to create ozone. The ozone molecule is also unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of oxygen and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle, thus creating an ozone layer in the stratosphere, the region from about 10 to 50 kilometers (33,000 to 160,000 ft) above Earth's surface. About 90% of the ozone in our atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 20 and 40 kilometers (66,000 and 130,000 ft.), where they range from about 2 to 8 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only 3 millimeters thick.