All forms of life need it, and if they don't get enough of it, they die. In some places, it's treasured and incredibly difficult to get.
In others, it's incredibly easy to get and then squandered.
Plants produce carbon-14 through photosynthesis, while animals and people ingest carbon-14 by eating plants. Scientists determine the ages of once-living things by measuring the amount of carbon-14 in the material.
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Many of the products that you use every day contain it or were manufactured using it.
Other Greek philosophers -- most notably Euclid and Ptolemy -- used ray diagrams quite successfully to show how light bounces off a smooth surface or bends as it passes from one transparent medium to another.
Arab scholars took these ideas and honed them even further, developing what is now known as geometrical optics -- applying geometrical methods to the optics of lenses, mirrors and prisms. Ibn al-Haytham identified the optical components of the human eye and correctly described vision as a process involving light rays bouncing from an object to a person's eye.
These different versions of elements are called isotopes, and small quantities of radioactive isotopes often occur in nature.
For instance, a small amount of carbon exists in the atmosphere as radioactive carbon-14, and the amount of carbon-14 found in fossils allows paleontologists to determine their age.