But outside the science departments and the textbooks, these categories all blend together. When we try to understand nature we find that everything is connected. Nature is usually too complicated for us to analyze from the viewpoint of more than one of the science disciplines at a time, except at a superficial level. But these life-giving forces also have the potential for destroying life. Our biosphere is the result of a balance between these opposing potentials.
Sunlight is delivered in small packets of energy called photons.
When an ultraviolet photon leaves the sun heading in our general direction, there is a small but real chance that it will collide with a DNA molecule in someone's skin cell and increase the likelihood that he or she will develop a skin cancer. The chance is small, but the number of photons is extremely large, so thousands of people develop skin cancer every year. Ultraviolet photons are produced whenever something is heated to a high enough temperature. This can be hydrogen on the surface of the sun or a piece of tungsten wire in a bright light bulb. Photons from the sun must pass through the earth's atmosphere before they can have any effect on us.