Light

Light is energy from the Sun that we can see. Light is essential to all life on Earth, as it is the source of food, air, and warmth. Visible light is actually made up of a spectrum of colors.

Although all life on Earth depends on light, it is easy to take it for granted. However, a world without light is almost impossible to imagine. Without light from the Sun, people's eyes would not work. Also, plants would not make their own food, which feeds other animals. Nor would plants give off any oxygen as a by-product of making food, and there would be no breathable air. Without light, there would be no warmth, and Earth would be as cold as the deepest part of outer space. On Earth, therefore, light means livable conditions and life itself.

All light is really energy that travels through space from the Sun. Sunlight is a form of energy called electromagnetic energy, or electromagnetic radiation. Physicists (a person specializing in the study of matter and energy and the interactions between the two) have long known that there are many kinds of this radiant energy that streams from the Sun in waves. The visible light from the Sun is only one type of radiant energy. The other types of radiant energy are known as gamma rays, x rays, ultraviolet, infrared, microwaves, and radio waves. The entire range of this energy, including visible light, is called the electromagnetic spectrum.

Each of these forms of radiant energy travels in waves and each has its own wavelength. A wavelength is the distance from one wave peak to the next. Gamma rays have the shortest length, while at the opposite end of the spectrum are radio waves, which have the longest length. Visible light is somewhere in between these two and is the part of the electro-magnetic spectrum to which the human eye is sensitive. Although this light appears white to the average person (some may say it is clear), it is really made up of another spectrum, a spectrum of colors. Thus the visible spectrum actually includes all the colors of the rainbow.

Finally, when this light streaming from the Sun encounters matter, the light is either reflected, absorbed, or transmitted to someplace else. What determines this is the color, or pigment, of the matter the light meets. Different pigments absorb different parts of visible light. The color of a pigment is determined by the type of light that it reflects or transmits. Thus, green pigment looks green to human eyes because it transmits and reflects green light. It also absorbs red and blue light, which we do not see. A black pigment absorbs all visible light, while a white pigment reflects all colors of visible light.

One of the great discoveries of science is the first law of thermodynamics. It is also called the law of conservation of energy. It states that energy can be neither created nor destroyed, although it can be changed from one form to another. This changing of the form of energy is what enables light to play such an essential role in the maintenance of life on Earth. Light energy from the Sun can be transformed into heat energy when it is absorbed by Earth. Even more important is the change from light energy to chemical energy. This occurs during photosynthesis.

Remarkably, plants absorb less than 1 percent of the sunlight that reaches Earth. This is enough, however, to allow every plant on Earth to grow and make food through the process of photosynthesis. This chemical process begins with sunlight. It carries out a chain of chemical reactions that produces not only food for the plant but oxygen for the atmosphere. When humans breathe the air and eat their food (animal or vegetable), they are incorporating the energy from the Sun (light) into their own beings. Light is therefore truly the source of all life.

[See alsoPhotosynthesis ]