Using Light

If an object gives off its own light it is a luminous object. Examples of luminous objects are the sun, light bulbs, candles, campfires and fire flies. Other objects can be seen because they reflect light. Objects which reflect light are called illuminated objects. Examples of illuminated objects are the moon, you, me, walls, desk and posters.

There are three ways in which luminous objects can be made to give off light. The first way is to heat objects until they glow. Light produced from heat is called incandescent light. Examples of incandescent light are ordinary light bulbs, the sun, a campfire and a candle. The ordinary light bulb is called an incandescent bulb. The incandescent bulb has a thin wire (called a filament) which is made of tungsten. Tungsten can be heated to 2,000 degrees Celsius without melting. Electricity is electrons flowing along a conductor. As the electrons flow through the thin filament, resistance produces heat causing the filament to glow.

The second way that light can be created is by bombarding gas molecules with electrons. These are called fluorescent lights. We have fluorescent lights in our classroom. Fluorescent light bulbs contain a gas which is under low pressure. The gases which are usually used are mercury vapor with argon. When bombarded with electrons, these gases give off ultraviolet light. We cannot see ultraviolet light. However, the inside of a fluorescent bulb is coated with phosphors. Phosphors absorb the ultraviolet light and glow. We see the glow of the phosphors.

The third way that light can be created is by using gases such as neon, which when bombarded with electrons, produces a visible glow that does not require phosphors. This type of light is usually called a neon light, even though other gases besides neon are often used. Neon produces a bright red glow. Other gases can be added to neon to produce other colors. Mercury vapor, without argon, produces a greenish-blue light which does not produce a glare. These are often used as street lights. Sodium vapor lamps give off a bright yellow-orange light. Sodium vapor lights do, however, produce a glare. Despite the glare, sodium vapor lights are also used as street lights because they use less electricity than mercury vapor. Both fluorescent and neon lights use less electricity and produce less heat than do incandescent light bulbs.

Light can have many uses. Laser light, for example, is used in to do surgery, to play CD's, in videos, computers, printers, and to put on entertaining light shows. Ordinary light sources produce white light. White light is a mixture of all frequencies of visible light (all colors). A laser is made up of only one frequency of light. Laser light is also coherent light. This means that the crests and troughs of the light waves all move in the same direction at the same time. Because it is coherent light, laser light does not spread out, but moves in parallel lines. Therefore, the definition of laser light is an intense beam of coherent, single color light. Lasers are constructed of gases, liquids and crystals. They also use mirrors. Lasers can be used to produce holograms. Holograms are the use of laser light to produce a three dimensional image. Holographic systems are used to scan the universal bar codes which appear on items for sale at stores. Holographic systems are also used to detect structural flaws in machine parts, to display the interior of organs of the body and to produce a three dimensional television image. The bar codes on products identify the manufacturer, the product, the size, as well as the cost of the product. These codes are read by lasers.

Another use for light is in fiber optics. Fiber optics use the principles of laser light and refraction to make a light pipe or a light wire. The pipe or wire is made of flexible fibers of glass or plastic called optical fibers. Fiber optics can be used in communications to transmit telephone calls, television signals and for data processing. Fiber optics are also used medically to travel through the body and transmit pictures of things like blood vessels, the stomach, the intestines, and the heart.

A prism is usually made of glass or plastic and is triangular in shape. As light passes through a prism, it refracts (bends) as it enters one side, and refracts again when it comes out the other side. Since the different wavelengths of light do not bend equally, they separate. Red light bends the least, violet bends the most. Because of unequal bending, the colors separate into the individual colors of the visible spectrum. The colors of the spectrum can be easily remembered as ROY G BIV--red, orange, yellow, green, blue, indigo, violet.

A rainbow is caused by the refraction of light through water. Because of unequal bending, the colors separate. To see a rainbow, the sun must be behind you and the air in front of you must be moist. Rainbows can be created using a garden hose. To do this, the sun must be behind you while spraying the mist in front of you.

If you look at the colors of the spectrum from a prism or a rainbow, the blue parts (blue, indigo and violet) occupy a larger part of this spectrum. This is because the blue end of the spectrum is light that bends more than other colors of light. The sun emits white light (light having all colors). However, because blue light bends the most, as it goes through the air, water and dust of our atmosphere, blue light spreads out, or scatters more than other colors of light. Because of this, more blue reaches our eyes. This makes the sky appear to be blue.

In the morning or evening the light of the sun must pass through more atmosphere to reach our eyes. This additional air, water and dust causes the green, yellow, blue, indigo and violet to scatter so much that it gets absorbed by the air, water and dust. All that reaches our eyes is the red and orange. Therefore, a sunrise and sunset cause the sky to appear red or orange.

The scientific model for light is that light is both a wave and a particle called a photon. As light hits matter, the electrons of matter absorb the photons. This energy causes the electrons to jump to a higher energy level. Since atoms want to keep their electrons in the ground state (lowest energy level), they give off or get rid of the photon quickly. However, some elements can keep these photons for a longer period of time, and their electrons stay at a higher energy level for up to one hour. These substances are called phosphorescent. Phosphorescent substances are used to make things (such as watch dials) glow in the dark.

Using Light Study Sheet