Shut your eyes tight—all you can see is darkness.
Now open your eyes again. If it is daytime, you will see that light is all around you.
What is Natural light?
Anything that gives off a light that we can see is called a light source.
The light sources that are not created by people are called natural light sources. They include the sun and the stars.
During the day, the rays of the sun light up the earth.
Sometimes, if there are no clouds, you can see the stars twinkling in the sky at night.
There are other sources of natural light, too. In some parts of the world, you can find insects called fireflies. They give off a pale, greenish-yellow light that flashes or glows in the dark.
Even deep down in the sea, some of the fish are able to produce flashes of light in the darkness. And if you live in the extreme northern or southern regions of the world, you might be able to see the aurora, a dazzling display of colored lights that flicker in the sky at night.
How does a light bulb convert energy?
There are also many light sources that don’t occur naturally but are created by people.
These are called artificial sources of light.
Electric lights, oil lamps, and even candles are all artificial light sources. You can find your way in the dark by using a flashlight powered by batteries.
Television and movies are made by using light.
And city streets are often full of artificial lights—vehicle headlights, brightly colored advertisements, street lights, and neon lights.
Light does more than just enable us to see. We use beams of light to cut metals into complicated shapes, or to perform delicate surgical operations! Light even helps us to remain healthy.
When sunlight shines on our skin, our body makes a vitamin called vitamin D, which helps our teeth and bones to grow healthy.
Light and heat
Take a look around the room you’re sitting in. How many different things can you see that are able to give off, or emit, light? You could do a survey of the light sources in your home. Make a list of the light sources in each room of the house. Against each item on the list, write the color of the light and whether it gives off heat as well.
You will probably find a lot of different objects that emit light at home. Do most of these give off heat?
You will see that the items on your list don’t all emit light of the same color. The flame of a gas cooker is blue, and the light from a flashlight is yellowish-white. A flashlight bulb shines a dull orange as the batteries run down.
Place your hand about 2 inches (5 centimeters) away from an electric light bulb, being very careful not to touch the bulb! Can you feel the heat that the bulb gives off? The bulb is using electricity, or electrical energy, to produce both light and heat. The light from the bulb is being used, but the heat is not used—it is wasted energy.
Another kind of light called a fluorescent light is used to change more of the electrical energy into light and less into heat. Try holding your hand close to fluorescent light. Does it feel as hot as the ordinary light bulb?
How does light travel?
Light travels in straight lines. It always travels like this unless it meets something that makes it change direction. One way in which light can change direction is by reflection. Can you think of an object that reflects light? We see the things around us because of rays of light that are reflected into our eyes. Light rays travel in straight lines from the object at which we are looking and into our eyes.
The thin beam of strong light travels in a straight line until it hits this hand. This light is reflected off the hand and then travels in a different direction. Crossing the road would be easy!
Can light travel around corners?
Imagine what it would be like if light could travel around corners! You would be able to see what is happening upstairs in the bedroom when you are standing downstairs in the kitchen. You could also see around a bend in the road. Crossing the road would be easy!
Stand behind a doorway and hold up a mirror to see what is happening in the next room. Light rays reflected from objects in the next room are reflected again by the mirror into your eyes.
This and other diagrams in this book show light rays as straight lines. The arrows indicate the direction in which the light travels.