LIGHT

1. LIGHT Duane M. Paloma III-Galileo

2. LIGHT - is so common that we often take it for granted. Without light, we would have no food to eat or air to breathe. - gives us various types of fuels we use today. Dead plants were changed into coals, natural gas, and oil. -from the sun also heats the earth. Without sunlight, the earth would become so cold that nothing could live on it. -an electromagnetic wave, a form of energy that can travel freely across space.

3. EARLY IDEAS ABOUT LIGHT • Corpuscular Theory The understanding of light has developed mainly since the 1600’s. Sir Isaac Newton discovered that white light is made up of all colors. Using a prism, he found that each color in a beam of a white light could be separated. Newton proposed the theory that light consists of tiny particles that travel in straight lines through space. He called these particles, corpuscles, and his theory became known as the corpuscular theory.

4. Wave Theory About the same time that Newton proposed his theory of light, the Dutch physicist and astronomer Christiaan Huygens suggested that light consists of waves. He proposed the wave theory to explain the behavior of light. The corpuscular and wave theories appear to be completely opposite, and scientists argued about them for about 100 years. Then, in early 1800’s, the English physicist Thomas Young demonstrated the interference of light. He showed that two light beams cancel each other under certain conditions. Water waves also behave this way. Because it is hard to understand how interference could occur with particles, most scientists accepted Young’s experiment as proof of the wave theory of light.

5. The Electromagnetic Theory In 1864, the British physicist James Clerk Maxwell proposed the mathematical theory of electromagnetism. According tot his theory, the influence that changing electric fields and magnetic fields have on one another allows for the travel of waves. Maxwell’s theoretical waves had the exact mathematical properties that had been measured for light. The vibrating electrical charges that produce light are the electrical charges in the atom. Atomic physicist had already shown that these vibrating electric charges exist. Maxwell’s work gave the wave theory a light of solid foundation.

6. Maxwell’s electromagnetic theory also did away with an idea that had stood in the way of scientists’ acceptance of the wave theory for more than a century. Scientists felt they had to find the medium(material) through which light waves travel. They reasoned that of light travels as waves, there must be something for them to travel through, just as sound waves need air to travel through. But for light, this something could not be matter, because light can travel in a vacuum. To get around this difficulty, scientists suggested that the medium light traveled through was the ether.

7. All attempts to observe or measure the properties of the ether failed. Scientists became increasingly convinced that the ether did not exist. Experiments conducted by Albert Michelson and the American physicist Edward Morley in 1887 helped destroy the ether theory.

8. Quantum Theory of Light In 1900, the German physicist Max Planck discovered an equation that matched experimental data about the emission of light by a hot surface. Planck could not explain why the equation worked. But he realized that it predicted that the tiny emitters of light on the surface can have only certain value of energy. When energy is restricted to certain values, it is said to be quantized.

9. In 1905, Einstein revealed that light itself is quantized. Einstein reasoned that if light emitters can have only certain values of energy, then the energy they emit as light will retain its quantized character. The light comes in tiny packet of energy that are known as quanta. The concept of light as quantized energy explained how light behaves as a particle in certain experiments, instead of as a wave. These particles of light came to be called photons.

10. In 1913, the Danish physicist Niels Bohr proposed that the energy of atoms was also quantized. When energy is given to an atom, either by collision or by shining light on it, the atom can accept only certain values of energy. One way it can do this is by emitting a photon that carries the energy away. Each type of atom accepts a different set of energies. Thus, when atoms emit light, the photons from one type of atom differ in energy from the photons from other types of atoms.

11. A field of physics known as quantum mechanics is the study of how atoms and light are quantized. It involves the fact that light and matter behave as waves in some experiments and as particles in other experiments.

12. Photons In 1905, the German-born physicist Albert Einstein proposed a model of light just as useful as the wave model. In some experiments, light behaves as though it is a particle. We now call this type of particle a photon. In Einstein’s model, a ray of light is the path taken by a photon. For example, when a flashlight sends a beam of light across a dark room, the beam of light consists of a great number of photons, each travelling in a straight line.

13. THE NATURE OF LIGHT There are many kinds of radiant energy, including infrared rays, radio waves, ultraviolet rays, and X-rays. We can see only a tiny part of all the different kinds of radiant energy. This part is called visible light or simply light.

14. SOURCES OF LIGHT Light makes it possible for us to see. Many of the things we see, such as the sun, a flashlight, and room lights, are sources of light. We see all other things because light from a source bounces off them and travels to us.

15. Light Sources can be classified in to two types: • Natural light- comes from sources that we do not control. Such sources include the sun and the stars. • Artificial light- comes from the sources that we control. These sources include candles and flashlights.

16. HOW LIGHT IS PRODUCED All light comes from atoms. It is produced by atoms that have gained energy either by absorbing light from another source or by being struck by other particles. An atom with such extra energy is said to be excited. Ordinarily, an atom stays excited only briefly. It de-excites by giving up its extra energy. It can either run into another atom to lose the energy, or it can emit (give-off) light. The light then carries away the extra energy. The amount of energy needed to excite atoms and the amount of energy the atom emit as light varies for different kinds of atoms.

17. Light is usually described as a wave, shaped much like a water wave that moves across a lake. But light can also be described as small particle, called a photon. Each photon moves in a straight line, much as a pool ball does. In both descriptions, the light has energy. The amount of energy that is carried by the wave or photon largely determines the color of the light. For example, suppose you see a red apple on a blue chair. Each photon from the apple has less energy than a photon from the chair.

18. One way to excite atoms so that they emit light is by heating them. A poker may be heated until it is white-hot. Because of the heating, the atoms at the pokers surface collide violently with each other. When they collide, they excite one another. Each atom quickly emits its extra energy as light but it is almost immediately re-excited by another collision. These collisions produce such a variety of states among the atoms that the photons released have a wide range of energies.

19. The combination of all the resulting colors is white light. As the poker cools, fewer atoms are excited to high energies, and so the atoms emit fewer photons with the higher energies of blue light. Since red light is still being emitted, the cooling poker looks red.