Light and Color

## What is Light?

Light is energy that moves in tiny particles known as photons. Light waves are very similar to sound waves, but can move much faster. Sound travels at about 1090 feet per second or 330 meters per second while light travels at 186,000 miles per second or 299792.458 kilometers per second. In equations, the speed of light is often written as the letter C. Another difference between light and sound waves is that while sound must have some form of matter in which to travel, such as air or water, light can travel through the vacuum of space. That is how light from the sun gets to us or how we can see the light from distant stars.

## Wavelengths

Light waves have wavelengths. That is the distance between the crest (high spot) of one wave and the crest of the next wave.

Each kind of light moves with a different wavelength. Some, like X-rays, move in short wavelengths. These waves have a huge amount of energy. UV waves from the sun are another example. If we get too much exposure to X-rays or UV waves, our skin can be damaged. We can get sunburned or worse when overexposed to the UV waves in sunlight.

Visible light moves at a more medium wavelength - that is why we can see it. But even visible light has many different wavelengths within it. More on that later.

Then there are the longer wavelengths. These include radio and television waves and also microwaves. You might even have a microwave oven to cook with in your kitchen.

Longer and shorter wavelengths are not visible to our eyes without special equipment. Only visible light moves in such a way that we can see it.

## Visible Light

Every kind of light moves at a certain wavelength. If each wavelength was laid out on a chart, it would create what is called the electromagnetic spectrum. The shorter waves would be on one end and the longer ones would be on the other. In the middle would be where visible light would end up. That is because it moves at just the right wavelength for our eyes to see it.

## Color

Within the visible light of the electromagnetic spectrum are still more wavelengths. Each wavelength is perceived by our eyes as a different color. The shorter wavelengths of visible light are violet - we might call them purple. Then as the wavelengths get longer and longer, the visible light changes in color to blue, green, yellow, orange, and finally the longest, which is red.

Some animals can see waveslengths of light that humans cannot. Those waveslengths would be just outside the edges of human visible light. For example, insects can see ultraviolet waves - waves just before purple on the electromagnetic spectrum. But we are not able to see these. At the same time, there are colors of red that insects are unable to see, but that humans can.

Most light that we use - like sunlight or light from a light bulb - is actually a mixture of all of the visible light wavelengths. It is called white light. When it is mixed, it is difficult for us to separate the colors out. We just see it as useful light. But at certain times the light does get separated out.

## Rainbows

The visible spectrum of light is often mixed together in what is called white light. We do not see each of the colors when they're mixed together. In order for that to happen, something must separate the wavelengths into their various colors. This can happen in a rainbow.

When light passes through certain materials such as water droplets from a storm or a sprinkler, the light can bend. If it bends just right (and it has to be just right), each of the different wavelengths can be seen. Because the bending has to be just right, sometimes you won't see the full arc of a rainbow. Some of it may be missing or even appear to be hiding inside of a cloud.

The colors in a rainbow appear to be red on the top and progress down through orange, yellow, green, blue, indigo and violet. Violet is another word for purple. Many scientists argue that there really isn't indigo in a rainbow. But tradition has it that the colors spell out an easily remembered name: *Roy G. Biv*. Next time you see a rainbow, see if you can spot the indigo - a kind of blue purple.

## Prisms

Prisms are another way that light can be bent. Prisms are actually a specially cut piece of glass or other clear material. If placed just right in a stream of white light, they can separate the light into its various colors. You may have played with one before. Sometimes other things can act as prisms such as the edge of a glass, a CD, or a piece of jewelry.

## How Light Behaves

Besides being bent by prisms or water droplets, light has the ability to change direction. Reflected light light that hits a surface and bounces off is called reflected light. We see objects because of reflected light. If the surface is slightly rough, some of the light scatters as it bounces off. If the surface is very smooth the light scatters less and you can see your image.

As light travels through air it can move rather easily, but if it gets slowed down by traveling through a piece of glass, it is refracted. A great example of this happens when you have a straw in a glass of liquid. The top of the straw appears to be in a different location than the portion of the straw that is in the drink. The glass and the liquid are refracting the light or slowing it down. It makes the image change. This science is used to make lenses for eyeglasses, microscopes, telescopes and other tools that aid vision.

## How We See Color

We don't need to have a rainbow to see a red hat or a green leaf. The colors don't need to be separated out by bending the white light. Instead, what is happening is the object itself is responsible for our seeing a color. The white light is separated in a different way. Some of the light is being absorbed into the object and some of the light is reflecting off it. The red hat is absorbing all of the colors of the white light except the red. The red color reflects off of the hat and travels to our eyes. We see red.

The green leaf absorbs all colors except green which reflects off and we see a green leaf.

Every object that we give a color name to is really absorbing all of the other colors and reflecting the color we see back to our eyes. If an object looks white, all of the colors are *reflecting* off it. If an object appears black, that means that it is *absorbing* all of the colors.

## How Light is Affected By Matter

Light tends to travel in a straight line. However, when it comes in contact with other matter, it can sometimes be affected by this matter. Take a look at these three kinds of matter:

### Transparent

A transparent substance allows light to pass with only a little interference. There is a slight bending as light comes through our atmosphere; that is what causes our sky to be blue. But light from the flashlight travels across a room pretty much the way it came out of the flashlight. Air and car windows are considered to be transparent.

### Translucent

A translucent substance allows light to pass through, but it scatters it just enough that images on the other side can not been seen clearly. Shower doors and plastic milk jugs are sometimes translucent.

### Opaque

An opaque substance is one no light can travel through. It stops the light altogether. Doors, walls, chairs, concrete, and wood - they are all opaque.

## Lasers

Lasers are special forms of light that are used to perform surgery, show movies, play music, read bar codes, make copies and so much more. But what is a laser? A laser is just one wavelength (one color) of light that has been specially focused so that it is concentrated to a pinpoint.

## Optical Illusions

Optical illusions have very little to do with light - they are more involved with our brains. But let's talk about them anyway. They often appear to be light-related. We see things that are not really there, or we see things in a way that we know cannot really take place but it seems like they do anyway. These are optical illusions. We enjoy them and have a fun time playing with them. But the truth is they are actually about our brain. Optical illusions are based on what our brain expects things to be or what our years of experience tell our brain that they should be. Sometimes it appears as if stationary objects move, straight objects curve, or fixed objects change size or color. Try this one to the right. Are the vertical lines angled? They are not - check them with a ruler.

## Color Wheel

The three colors known as the primary colors - red, yellow and blue - are used in televisions to create all of the colors that you see. If you were to mix any two of these colors, you would get the secondary colors - orange, purple, and green. These six colors are known as the basic color wheel. Artists, photographers, and others work with the color wheel to make things look pleasant. When the colors are arranged according to how they mix, patterns can be seen. For example, artists will often try to use opposite colors from the wheel in projects because opposites actually complement each other. Yellow and purple will go well together and even make each other seem brighter. When used this way they are called complementary colors.