LIGHT colors are absorbed This causes the tomato to look red Another aspect of light that has proved central to the study of physics is its speed Measuring the speed of light was a long standing challenge for scientists In 1905 Albert Einstein made a breakthrough As part of his theory of special relativity he put forth that the speed of light is the same for all observers Since then the speed of light has been considered a fundamental constant of nature In Einstein's famous relativity equa tion E mc the speed of light c serves as a constant linking the formerly separate con cepts of mass m and energy E The work of later scientists refined measurements of the speed of light which was eventually defined at exactly 299 792 458 meters 186 282 miles per second This volume explores the mysterious properties of light from its physical charac teristics to its intriguing behavior It provides a strong foundation for the study of this crucial and complex form of energy which fundamentally shapes our everyday lives and on the grandest scale provides a window on the structure of the universe 8
CHAPTER 1 THE FUNDAMENTALS OF LIGHT O ne of the most familiar and important forms of energy is light Nothing is visible to humans when light is totally absent But light is even more important for other reasons Many scientists believe that mil lions of years ago light from the Sun triggered the chemical reactions that led to the develop ment of life on Earth Without light the living things now on Earth would be unable to sur vive Light from the Sun provides energy for life on Earth Plants change the energy of sun light into food energy When light rays strike a green plant some of their energy is changed to chemical energy which the plant uses to make food out of air and minerals This process is called photosynthesis Very nearly all living organisms on Earth depend directly or indi rectly on photosynthesis for their food energy Some of the energy of sunlight is absorbed by Earth's atmosphere or by Earth itself IO
THE FUNDAMENTALS OF LIGHT Much of this energy is then changed to heat energy which helps warm the planet keeping it in the temperature range that living things have adapted to LIGHT AND ELECTROMAGNETIC RADIATION Different kinds of light are visible to different species Humans see light in what is called the visible range It includes all the colors begin ning with red and continuing through orange yellow green blue and violet Some people can see farther into the violet region or the red region than other people Some animals have a different sensory range Pit vipers for example have sense organs pits that see rays that humans feel as heat These rays are called infrared radiation Bees on the other hand not only see some of the colors that humans see but are also sensitive to ultravio let radiation which is beyond the range visible to humans So though human eyes cannot detect them infrared rays and ultraviolet rays are related to visible light Instruments have been built that can detect and photograph objects by means of infrared rays or ultra violet rays X rays which can also be used to photograph objects are also related to light II
