World Book Trivia Challenge--light-australia

Waves that have wavelengths slightly shorter than those of visible light are called ultraviolet rays. Ultraviolet light from the sun can cause sunburns and skin cancer, but many forms of ultraviolet light find use in medicine and technology. Although it causes sunburn, ultraviolet light does not feel warm. Infrared light is the form of light from the sun we feel as heat.

A transparent material, such as a calm pool of water, lets light pass through it with little scattering. We can see through such a material. A translucent material also allows light to pass through it, but the material scatters each wave in many directions. As a result, we cannot see clearly through such materials as frosted windows. An opaque material blocks all light.

Light seems to travel across a room the instant a light bulb is turned on. But while light does travel quickly, its speed is not infinite. The speed of light has been measured to be about 186,000 miles (300,000 kilometers) per second. That is fast enough to circle the Earth more than seven times in one second.

People have found ways of making and controlling light, enabling them to see when there is no sunlight. At first, they produced light with fire—using campfires, torches, and later candles and oil lamps. In the late 1800’s, they developed electric light bulbs. In the mid-1900’s, scientists developed tiny and efficient light sources called light-emitting diodes (LED’s).

Transverse waves travel through a material. For example, waves in water move through the water even though the water molecules themselves only move up and down. The molecules return to their original position after the wave has passed. Light waves, like some water waves are transverse waves.

One way to excite atoms to emit light is by heating them. The process by which a heated object gives off light is called incandescence. Incandescent light bulbs heat a small piece of metal, called a filament, to produce light.

Lasers rank as one of the most important and useful inventions based on light. Ordinary light bulbs give off light that consists of many colours (wavelengths) that spreads out in many directions. In contrast, lasers emit light with only one colour, or wavelength, and the light undergoes virtually no spreading. Light with these qualities is said to be coherent. Such light can travel great distances with little loss of intensity.

Modern study of light began in the 1600's. In 1666, the English scientist Sir Isaac Newton discovered that white light consists of all colours. Using a prism, he found that a beam of white light could be separated into the colour spectrum.

Waves with somewhat shorter wavelengths than ultraviolet rays are called X rays. These rays are powerful enough to penetrate a person's body. Doctors and dentists use them to "see" inside the body. Gamma rays have even shorter wavelengths than X rays. Gamma rays carry the most energy of any form of light. Light exposure to gamma rays and long-term exposure to X rays can be harmful to living things.

In the early 1000's, the Arab scientist and mathematician Alhazen wrote a book containing a detailed theory on the properties of light rays and their role in vision. For this work, Alhazen is sometimes known as the founder of optics, the study of light and vision.