The Old Testament begins: "In the beginning God created heaven and earth. The earth was unformed and void, with darkness over the face of the deep and a wind from God sweeping over the water. God said, 'Let there be light'; and there was light." (Genesis 1:1-3) By the Biblical account, the creation of light was one of God's first acts. Light is at the very foundation of life on Earth as we know it.

In Greek mythology, Apollo was the god of the Sun, logic, and reason. The Greeks had already made a connection between light and the workings of the mind. This concept exists today in phrases like "Let's shed some light on the subject" and "I see!" A standard convention in cartoons is to represent someone who has an idea as having a lightbulb go on over their head.

In Celtic mythology, there is a dark Holly King, disguised as a wren, and his twin, the light Oak King, disguised as a robin. At the summer solstice the Holly King kills his twin and begins a half-year reign. At the winter solstice, the robin slays the wren and he reigns for the next six months. There is an eternal duel of light and darkness.

It is easy to imagine how ancient peoples may have feared the darkness of night, not knowing if light would return. The passing of summer into autumn and the increasingly short days of winter must have also caused concern over whether spring would really arrive as in past years.

Light is such a central component of human life that it is probably no coincidence that many religious and mythological texts try to explain it. Also some of the earliest scientific work by cultures such as the Egyptians and the Babylonians dealt with light in the context of astronomy.

• Christmas is marked by decorations of light — hanging in town squares and streets, on people's homes, and on their Christmas trees. Candles are common as well, in remembrance of "the star of Bethlehem."
• The Jewish holiday Channukah — also called the Festival of Lights — falls in the month of late November or December. Celebrants light candles in a special stand called a menorah. Starting with one candle on the first night, an additional candle is lit each night for a total of eight nights.
• Kwanzaa, an African American holiday, is celebrated from December 26 to January 1. Although this relatively recent holiday was created in 1966, it also includes light, as celebrants light candles in a kinara each night.
• St. Lucia Day is celebrated in Sweden on December 13, the longest night of the year. The origin of the connection between this fourth-century Sicilian saint and Sweden is unknown. St. Lucia is often depicted with candles on her head, and tradition associates her name with light. Celebrants wear a wreath with a single candle.
• The Hindu festival Diwali derives its name from a Sanskrit word meaning "row of lights." The holiday falls in late October. Celebrants place small oil-filled lamps along the parapets of temples and houses and also float them on rivers and streams.
• The Chinese Lantern Festival is celebrated in the middle of the first month of the lunar calendar, sometime between late January and late February. It closes the Chinese New Year celebration. An ancient Chinese myth depicts celestial spirits flying about in the light of the first full moon of the lunar calendar. Celebrants hang colorful lanterns at temples.

The symptoms of SAD are:

• sadness, irritability, even violence — the number of suicides and cases of child abuse increase during the winter months
• decreased physical energy — SAD victims often feel sluggish when awake and want to sleep more hours than usual
• an increase in appetite — especially a craving for carbohydrates, possibly the body's attempt to increase its energy level

Individuals most susceptible to SAD are women, people who live in northern latitudes, people with a family history of mood disorders, and people who work or study in windowless rooms. SAD victims usually feel the onset of symptoms in the fall and feel relief setting in by the spring.

Light therapy has become a standard treatment for SAD. Doctors prescribe exposure to a light box in the morning. "It's not the kind of light, it's the intensity," explains Dr. Norman Rosenthal of the National Institute of Mental Health.

Generally the light boxes use a bright light with a frequency spectrum closely simulating the frequencies of natural sunlight. The required length of exposure depends on the severity of the winter depression. While no one is certain why this helps, scientists hypothesize that it has to do with shifting the body's internal clock. And there are many testimonials to the success of light therapy.

• Sunlight is the source of all biological energy on Earth. Plants turn light into chemical energy in the form of sugars, which fuel the entire food web. Learn about light in the photosynthetic process in the Biology Gateways activity, The Role of Light in Photosynthesis. Then see if you can draw a connection between the photosynthetic process and people's craving for light and carbohydrates during the winter. (This and the other SimLibrary activities mentioned below require Logal Express. Get a free trial subscription.)

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Laser light shows are a balance of technology and art. They combine lasers, colored filters, mirrors, oscillators, projectors, and computers. These elements are the tools of a laser light show; how they are used is the artistry.

Laser shows depend on a phenomenon called "persistence of vision." Think for a moment of how a movie is projected. An individual frame is run quickly over a light source and projected onto a screen. The light-sensitive cells of the eye's retina transmit signals to the brain, which interprets the signals into an image. While this is occurring, the next frame already appears on the screen. The brain merges the various signals into one continuous image, smoothing the motion from one frame to the next. This phenomenon is also the basis of many optical illusions.

There are two major types of laser light show effects:

1. Beam effects, where the laser beam itself travels through the air. The beam may be held steady and just turned on and off, or it may be moved about to create interesting effects. For instance, the beam can be moved so quickly, that you see the outline it traces as an actual picture.
2. Screen effects, which are projected onto a screen or other surface, such as a dome. Screen effects may include cycloids and animations. Cycloids are combinations of circles, similar to pictures made with a Spirograph™. Combining circles of different speed, size, and direction creates patterns of great complexity.

Laser animation may be traditional character animation, where each frame is digitized from drawings prepared by an animator. Laser shows can also use object animation, in which the images are created by the movement or rotation of a projected object. Computers can also add depth to a laser light show by dimming parts of a projected object that should be perceived as being "farther" away.

You can find out more about lasers from these Web sites:

• How Stuff Work's Lasers gives a general introduction to how lasers work.
• The Laserium Web site has a gallery of images made with lasers, including space shuttles, clowns, and elaborate patterns.
• Perform "hands-on" activities with simulated lasers and an assortment of optical tools in Riverdeep's Virtual Labs: Light. You can investigate the nature of light, reflection and mirrors, refraction and lenses, and the colors of light. Try a free demo now.
• Laser Show Resource Guide: Provided by a commercial company, Pangolin, this resource guide provides technical information about the makings of a laser light show.