Light Energy & You

            Light is a form of energy. Without light you would not be able to see anything. You would have a very different idea of what your world is like if there were no light energy to help you see.
            In our everyday living, light from the sun or from some other source is with us practically all the time.
            One of the greatest discoveries was the use of fire. At first fire may have come from wood which had been set burning by lightning. Long ago, people probably did not know how to start a fire themselves. So when a fire was set, it was very important for them to keep the fire burning.

What is Light?

   For many centuries light was produced by the burning of some material such as oil, wax, or gas. Heating produces light. If an iron poker is left in the fire for some time, it becomes red and begins to glow. We say it is red hot. Blacksmiths found that heating iron to a higher temperature yet gave off a yellowish light…and even at a higher temperature produced a white light.
             As the electrons of the iron atoms receive more heat energy, they begin moving faster and faster. The more heat the electrons receive, the faster they move. The electrons now have more energy than they had before. The electrons give off this energy as light.
             Electrical energy heats wire as it moves through it. Electrical energy changes to heat energy in the wire. The atoms of the metal of which the wire is made become heated. When the electrons of the atoms receive a certain amount of heat energy, they give off light. Electrical energy is really being changed to light energy. This is the same kind of thing which happens in the electric-light bulbs which you use in your home.
         Men who study about metals tell us that the color of the light is different for different metals. It has been discovered that each metal gives off its own particular colors when heated to very high temperatures. Scientists have found that this is very useful information. They can place a bit of metal in a flame and notice the color of the light it gives off. The color of the light will help them to know what the metal is.

Light from the Sun

            About 90% of the sun is made of hydrogen gas. Atoms of hydrogen gas in the sun join together, and a new gas is formed. When four atoms of hydrogen gas combine, they form an atom of helium gas. Great amounts of energy are released when this happens.
            All the atoms of the sun are heated to very high temperatures. The atoms are heated by the energy which is released when helium gas is made from hydrogen gas.
             In the sun, in addition to the atoms of hydrogen and helium, there are atoms of other materials, such as iron, oxygen, carbon, and many others. The electrons of atoms of metals and of some gases give off light energy when heated to high temperatures. The electrons of the heated atoms in the sun give off light energy in the same way.
            Light energy from the sun strikes our earth. The sunlight travels about 93 million miles from the sun to our earth. Light travels at the speed of 186,000 miles in one second. From outer space we would be able to see the sun and the other stars shining in the sky. But the space around us would not be light. There are small particles in the earth’s atmosphere. When the sun’s energy reaches our atmosphere, it strikes these particles, and we see its light. There are few objects in the way of light energy as it travels from the sun to the earth. Most of outer space is thought to be empty. Since there are practically no particles for the energy to strike, outer space is black.

The Colors of Light

            A beam of sunlight is really a bundle of light. It is made up of many different colors of light. When sunlight strikes a piece of cut glass, the colors in the bundle of light change direction. As they go through the glass, they all change in the same direction, but some change more than others. Violet changes direction more than the other colors do. Blue changes direction more than green. Green changes more than yellow does, and yellow more than orange. Red light changes direction less than any of the others. In this way the bundle of light is separated into its various colors.  
            The color of objects depends upon the light that strikes them. When an object looks red or blue or purple or any other color, we say that it reflects that particular color. When light strikes a colored object, the object may send back only a small portion of the light. A red dish looks red because it sends back only the red part of the light which strikes it. If there were no red in the light, the dish would look black.  It would send back hardly any light at all.
            There are many different colors in the bundle of light coming from an electric light. When you look at a blue dress by electric light, you can see the color blue because there is blue in the light coming from the electric-light bulb. The dress might look a different shade of blue in the sunlight. There is usually a different amount of blue in electric light than there is in sunlight.
            Black paper does not reflect light. Instead of bouncing, most of the light sinks into the black paper. The light is absorbed.

How Light Travels

            There are two different theories used to explain the nature of light. These are the particle theory and the wave theory. Neither theory alone completely explains the behavior of light. The most satisfactory explanation is a combination of both theories. The particle theory of light says that light consists of tiny particles of energy called corpuscles. The wave theory of light says that light consists of waves that travel from a source.
            The particle model of light is in keeping with the fact that light travels in empty space, whereas sound cannot. Light usually travels in straight lines. When light particles strike an object, they bounce back from it, or are reflected. Light particles reflected from an object can form an image of the object on another surface. That is how we see. Light reflected from an object enters our eyes through the pupils. An image of the object is formed on the retina which is at the back of the eye. The retina acts something like a screen. Nerve messages from the retina reach the brain and we see the object.
            A mirror reflects light so well that it will show clear images of objects. The angle at which a light particle bounces off a surface is equal to the angle at which it hits the surface. When many light particles strike a very smooth surface, they are reflected evenly. A clear image can be formed. When light particles strike a rough surface, they are reflected in many different directions. The light is scattered and a clear image is not formed.  We say that the light is diffused. Light particles are very tiny. A roughness or irregularity that you could hardly feel might be quite big in comparison to a light particle. Most surfaces do not reflect images.

Light in Different Substances

            Light does not move at the same speed in all substances. It travels fastest in empty space. It travels faster in air than in water or glass. Light bends as it goes from one substance to another. The bending is caused by the change in speed.
            There is a perpendicular line where light strikes the surface of water. The perpendicular line is called the normal. When light moves from water to air, it bends away from the normal. When it goes from air to water, it bends toward the normal. The bending of light when it enters a different substance is called refraction. Refraction depends partly on speed. Fast-moving particles bend less than slow-moving ones. Light bends more when it enters substances which slow it down more.
            Imagine a beam of light entering a darkened room through a slit. This slit is blocked by a screen with a tiny hole in it. The light is allowed to fall on a wall. Francesco Grimaldi, an Italian scientist discovered that the light was bent by a small amount as it passed the edges of the hole in the screen. The bending of light as it passes a sharp edge is called diffraction. Diffraction is quite different from refraction. Light does not pass from one substance to another. According to the particle model, light should always travel in straight lines within a single substance. It should not bend around sharp edges. But in diffraction it does bend around sharp edges.
            According to the wave model of light, light travels out in all directions from its source in a wave motion. Each wave has a high point, called a crest, and a low point, called a trough. When there is diffraction, the waves do not bend as sharply as when there is refraction. Diffraction is one behavior of light that can be explained by the wave model and not by the particle model.
            Sometimes light acts as if it is made up of waves. At other times light acts as if it is made up of particles.
            Both the particle theory and the wave theory can explain reflection and refraction of light. The wave theory can also explain diffraction, but the particle theory cannot. However, the particle theory can explain how light can travel through empty space, and the wave theory cannot.