Light

Light

When you get up in the morning, sunlight makes your room bright. When the Sun sets, your room gets so dark you might trip over the furniture. Without light, you cannot see. So you flick a switch on the wall and electric light makes your room bright again.

You know that you cannot see without light. But did you know that life on Earth would not be possible without light? Light from the Sun makes Earth warm enough for life. Light from the Sun makes green plants grow. Plants are food for animals. Plants also give off oxygen that you breathe in the air. Dead plants provide fuel that we use for energy. Plants that died millions of years ago turned into coal and oil that people now use to make heat and electricity.

WHERE DOES LIGHT COME FROM?

Most light comes from things that give off heat. Natural light comes from the Sun, other stars, and certain animals that glow in the dark. Artificial light comes from things that people make. The flame of a candle gives off artificial light. Electric light bulbs give off artificial light.

Both natural and artificial light starts inside atoms, tiny pieces of matter much too small for anyone to see. Changes inside atoms give off light.

HOW LIGHT TRAVELS

Light travels at very high speed. It travels through space faster than anything we know of. You can tell that light travels faster than sound when you see lightning. You’ll notice that you usually have to wait a few seconds after the lightning bolt to hear thunder. Thunder is the sound that lightning makes.

The speed of light through empty space is about 186,000 miles per second (about 300,000 kilometers per second). Astronomers talk about vast distances in space as light-years. A light-year is how far a beam of light can travel in one year, or about 5.8 trillion miles (about 9.5 trillion kilometers).

Sometimes light seems to travel as waves. You can see waves ripple through water. You can see waves travel back and forth in a rope that you shake up and down. Most waves must travel through something, but light waves can travel through empty space.

Sometimes light seems to travel as little bundles of energy. The bundles are called photons.

COLORS OF LIGHT

Sunlight is made up of every color in the rainbow. When sunlight is spread out into all its colors, it makes what scientists call a spectrum. A rainbow is a spectrum. In a spectrum, each color flows into the color next to it.

You can think of light rays as strands of color. Each colored strand, or ray, has energy. The color of a ray of light depends on how much energy it has. Violet has high energy. Red has lower energy. The colors between violet and red make up the light we can see. This light is called visible light.

Beyond violet are ultraviolet light rays that you cannot see. Ultraviolet rays from the Sun and from artificial light can cause sunburn. Infrared light rays are beyond red. You cannot see infrared rays, but you can feel them as heat.

LIGHT BOUNCES

Light travels in a straight line until it runs into something. What happens then depends upon what the light runs into.

Light rays go right through transparent substances. Clear window glass is transparent, so sunlight goes right through. That is why you can see through a windowpane.

You cannot see through some things because light rays bounce off them. We see houses, chairs, our friends, and other objects in the world because light bounces off of them. This is called reflection. When light rays hit the smooth surface of a mirror, they bounce off in a straight line. That is why you can see a reflection of your face in a mirror. Light rays bounce off every which way when they hit bumpy surfaces. That is why you cannot see your face in your friend’s hair but you can see the hair.

Things that don’t let light through are opaque. Opaque materials can also absorb, or “soak up,” some or all of the colors that make up light. This is why you can see colors. When sunlight hits a red dress, the dress soaks up all the colors in light except red. The red rays scatter and are reflected back. You see these rays as the color red. Materials that soak up all colors that hit them are black.

LIGHT BENDS

Light bends when it enters materials. You can see light bend when it goes from air to water. Light bends because it changes speed when it enters the water. Put a pencil at a slant in a glass of water. The pencil looks like it is broken in two at the surface of the water. The surface is where the light rays slow down and bend as they go from air to water. This bending of light is called refraction.

WHY IS THE SKY BLUE?

The sky looks blue because air scatters the different rays of colored light in sunlight. When light rays scatter, they go off in different directions. Air scatters blue light much more than any of the other colors in sunlight. Most colors travel directly from the Sun to the ground, but blue gets scattered all over, turning the whole sky blue.

At sunset, however, the sky sometimes looks red. At sunset, when the Sun is at the horizon, light rays have to travel longer distances and through more air to reach you. This extra air scatters blue light so much that the blue light gets absorbed or bounced into space before it gets to you. Most of the light left to see is red.

HOT AND COOL LIGHT

You know that hot things give off light. But did you know that some cool things can also give off light?

Light that comes from heat is called incandescent light. The Sun is blazing hot. Sunlight is incandescent light. An ordinary light bulb gets so hot that it could burn you. An ordinary light bulb gives off incandescent light.

Light that comes from cool sources is called luminescent light. Some kinds of chemicals and electric energy give off light without giving off heat. Chemicals in a fluorescent tube give off luminescent light when electricity goes through the tube. You can touch a fluorescent light bulb without getting burned.

The bodies of some animals have chemicals that give off light. The glow of a firefly at night is luminescent light.