The sun is the most abundant source of energy in the solar system. Every day, 173,000 tera-watts of energy—100,000 times more energy than humans use—strike the earth in the form of sunlight. Sunlight consists of visible light; ultra violet light, which is responsible for the production of Vitamin D in our skin; and infrared radiation, which heats the air, water, and ground, and constitutes nearly 50 percent of the total solar radiation reaching the earth. Without the sun, life on Earth would not exist. One would think that all that free, available energy would have been harnessed long ago, but only in recent decades have people taken a serious interest in the sun as a source of alternative renewable energy.

So far, the world has relied on wood, oil, coal, and natural gas to produce energy for heating, lighting, transportation, and manufacturing. Since the 1950s, the world consumption has been increasing relentlessly! In the year 2000, 80 percent of commercial energy was generated from the combustion of fossil fuels, and every day, 50 million barrels of oil are refined and converted to gasoline and other fuels. In addition to the rapid depletion of fossil fuels, the consequences of high consumption rates are pollution and global warming.

Rising concentrations of carbon dioxide (CO₂) in the atmosphere have been causing global temperatures to rise, with the greatest increase occurring in the last decade. In the 1990s, global CO₂ emissions rose by 1.3 percent per year. Between 2000 and 2006, annual CO₂ emissions increased to 3.3 percent, and since then they have been accelerating. To meet the 50-60 percent increase in energy demand by 2030, as predicted by the International Energy Agency, and to prevent the disastrous effects of global warming, people have been looking to the sun.

The most practical form of solar energy is passive solar heating, which occurs when natural light floods in through windows or is absorbed by stone or concrete walls. In 1956, the world’s first commercial solar building was constructed in Albuquerque, New Mexico, by engineers Frank Bridgers and Don Paxton. Considered ahead of its time, architect Francis Stanley’s design provided for large sloping south-facing windows to capture infrared radiation, following the same principles used by the ancient Romans to heat their bathhouses. Passive solar energy is now being incorporated into environmentally friendly buildings, using thick walls and ceilings to trap heat, Trombe walls painted in dark colors to aid the absorption of infrared radiation, and mirrors or fiber optics to enhance natural lighting.

Ultra violet light is responsible for the production of ______ in our skin.