Why does an Electric Bulb Give off Light?

A light bulb gives off light because an electric current is passed through its filament, a thread of tungsten metal thinner than a human hair, which then becomes white hot.

Sir Joseph Swan (1828-1914) in England and Thomas Alva Edison (1847-1931) in the United States constructed the first incandescent electric lamps in 1879. They succeeded in preventing the rapid burning up of the filament by oxidation, but their problem lay in the choice of a suitable material for the filament.

Edison sat in his laboratory watching a filament of charred cotton thread glow in a glass bulb, which had been exhausted of air, for 40 hours. But the thread was too fragile to sustain the heat provided by the electric current. A material was needed that would stand great heat, for the hotter the filament, the brighter is the light given by the lamp.

But even with the present day use of tungsten, the problem still remains in that the more a filament is heated, the sooner it will burn away. To solve this problem, gas discharge lamps were invented.

These consist of glass tubes filled with sodium or mercury vapor, or neon gas. At each end of the tubes are electrodes, or contacts. When an electric current is applied to one of the contacts it passes through the gas to the other contact, causing the gas to glow and give out light.

Why You Shouldn't Watch Television in a Darkened Room

You should not watch television in a darkened room because you will not see the picture so clearly. The human eye functions best and is able to see detail most acutely when viewing white light. So a lightish background makes for a better picture.

Of course, it is unwise to have the source of light shining directly on to the television set because some of the rays will be reflected and spoil the picture.

Viewing in a dark room can also cause an unpleasant glare. The intense light coming from a televsion screen against a black background causes a certain amount of stray light to be reflected internally from structures within the eye on to the retina. The retina is the delicate tissue containing the photo receptors.

The effect is similar, if less intense to that produced by a car with undipped headlights.

A third disadvantage of a darkened background is that "flicker" on the television screen becomes more apparent and can cause severe eye fatigue. In certain quite rare cases this "flicker" effect can produce epileptic fits.

What Is a Light Year?

A light year is a unit of measurement used by astronomers and represents the distance light travels in one year. This distance is approximately 6,000,000,000 miles, the speed of light being just over 186,000 miles a second.

Astronomers adopted this unit because they found ordinary measurements impractical for coping with the vast distances of space. Using this means, scientists calculate that the Milky Way must be about 100,000 light years in diameter.

The light from some stars takes millions of years to reach us. Andromeda is about 1,800,000 light years distant from earth, although it can be seen with the naked eye. Our nearest star, Proxima Centauri, is about four light years away, while the most brilliant star, Sirius (the Dog Star), is eight and a half light years from us.

So, when you look up at a star in the sky, you're like looking back in time.