Light & Color

Light is a mixture of all visible electromagnetic wavelengths; color is the sensation resulting from the eye seeing light waves of certain lengths.

Light is vitally important to us! Without light, there would be no life on our planet. Light is required for plants to grow and thus provide food for people and animals. Light makes it possible for us to see. Without light, there would be no color. There are many sources of light, including natural sources such as the sun, the stars, and lightning. Burning objects such as candles and bonfires give off light, as do certain animals such as fireflies and glowworms. There are also man-made sources of light, such as electric light bulbs and flashlights. Light is all around us, but what exactly is it? Let’s take a closer look at light

Light is a form of energy. What we call light is electromagnetic radiation that the human eye can see. It exists in tiny packets called photons. Scientists say that light has properties of both a particle and a wave. Light waves travel at a speed of about 186,282 miles (300,000 kilometers) per second. As far as we know, light is the fastest thing in the universe.

Our most important source of light energy is the sun. Because the sun is so far away, it takes about eight minutes for light from the sun to reach the Earth. By contrast, it takes about 1.3 seconds for light to go from the moon to the earth. Sunlight is powerful: it provides energy for plant photosynthesis, it gives our bodies essential vitamin D, and it can penetrate 262 (80 meters) into the ocean. Learn more about the sun at Science Trek’s Sun page.

Light waves are similar to sound waves, but there are some important differences. Light waves move much faster than sound waves. Another difference 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 and how we can see the light from distant stars.

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

The light we see, visible light, is only one part of the electromagnetic spectrum. The wavelengths of visible light are in the medium range. But there are other types of energy that have shorter or longer wavelengths. Each kind of light energy has a different wavelength, so the length of the wave determines the type of radiation energy it is. The shorter the wavelength, the more energy the light has.

For example, ultraviolet waves move in shorter wavelength than visible light. We can’t see ultraviolet (UV) waves, but we can see their effects. If you get too much exposure to ultraviolet waves from the sun, you can get sunburned or your skin or eyes can be damaged. X-rays, which can be used to photograph objects, also move in short wavelengths and have a huge amount of energy. On the other hand, infrared waves and microwaves have longer wavelengths than visible light and contain less energy. Radio waves have photons with the lowest energy.

Visible light moves at just the right wavelength for humans to see. But different kinds of light are visible to different species. Bees and butterflies can see ultraviolet waves, which are beyond the range visible to humans. Some snakes can detect infrared radiation, meaning that they can “see” heat emitted by their prey. Special instruments have been developed that allow humans to detect objects using infrared or ultraviolet light. For example, forensic scientists can use UV light to detect evidence the human eye cannot see, and infrared light is used in night vision technology.

Within the visible light portion of the electromagnetic spectrum, there are still more wavelengths. Each wavelength is perceived by our eyes as a different color. The shortest wavelengths of visible light are violet, while red light has the longest wavelength. Our eyes can detect red, yellow, and green light and many combinations of those colors. Learn more about how humans see color.

Different animals can see different wavelengths of light. Cats and dogs typically see only two colors, while birds see five to seven colors. Find out more about what colors animals can see and how their color vision compares to that of humans.

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, we don’t see the different colors. We just see it as useful light. But under certain conditions, the white light does separate out into its component colors.

The visible spectrum of light is usually mixed together and seen as white light. In order for us to see the spectrum of colors, 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 water sprinkler, the light can bend. If it bends in a certain way, each of the different wavelengths can be seen. The colors in a rainbow appear in order of their wavelengths: red, orange, yellow, green, blue, indigo and violet. The progression of the colors of the rainbow spell out an easily remembered name: ROY G. BIV.

Prisms are another way that light can be bent. Prisms are specially cut pieces of glass or plastic. If placed just right in a stream of white light, a prism can separate the light into its various colors. Sometimes other things can act as prisms such as the edge of a glass, a CD, or a piece of jewelry.

Light behaves in several well-known ways that scientists describe as reflection, refraction, and scattering.

Besides being bent by prisms or water droplets, light has the ability to change direction. Light that hits a surface and bounces off is called reflected light. The reason that we see objects is that light reflects off them into our eyes. If the surface of the object is slightly rough, some of the light scatters as it bounces off. If the surface is very smooth, the light scatters less. Mirrors have a smooth, metallic coating that reflects almost all the light that hits them. When you look into a mirror, light hits the shiny surface and bounces off into your eyes, and you see your own image. You might look at a polished teaspoon and see your face quite clearly, but if the spoon is dirty, the bits of food and dust scatter light in all directions and your face disappears.

We see objects because they either give off their own light, or light reflects off the objects and enters our eyes. The moon is an example of reflected light. Unlike the sun and stars, the moon doesn’t make its own light. We see the moon only because it reflects light from the sun.

As light travels through air it moves in a straight line, but if it gets slowed down by traveling through glass or water, it is refracted, or bent. You can see an example of this if you place 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 liquid. Because light slows down when it moves from air to water, the straw appears to be bent at the point where the air meets the water.

The science of refraction is used to make lenses for eyeglasses. The lenses in glasses are curved pieces of glass or plastic that bend or refract the light from objects being viewed. This curved lens directs the light rays into the eye more effectively, so the person with impaired vision can see clearly. A magnifying glass has a convex lens that bends light rays so the image appears larger. The principles of refraction are useful in making binoculars,  microscopes, telescopes, and other tools that aid vision. Besides being reflected and refracted, light can also be scattered when its travel is interrupted. As sunlight reaches the earth, it hits gas and dust particles in the atmosphere and is scattered in all directions. Blue light is scattered more than other colors because it has shorter, smaller waves. That is why we see the sky as blue.

Learn more about reflection, refraction, and scattering from the American Museum of Natural History.

The eye changes light rays into electrical signals that are sent to a special part of the brain called the visual cortex. The brain interprets these electrical signals as visual images. The eye is able to see in dim light or bright light, but it cannot see objects when light is absent. Without a source of light, the objects do not reflect light and disappear from our view.

What about seeing color? If white light includes all colors mixed together, how do we see a red hat or a green leaf? When we see any object, some of the light is being absorbed into the object and some of the light is reflecting off it. The red hat absorbs all the colors in the white light except the red. The red color reflects off the hat and travels to our eyes, and we see red. The green leaf absorbs all colors except green, which reflects off and we see the leaf as green.

Every object that we identify with a color name is really absorbing all of the other colors and reflecting the color we see back to our eyes. If an object looks white, all the colors are reflecting off it; no colors are absorbed. If an object appears black, that means that it is absorbing all the colors and reflecting no light. Learn more about how we see on Science Trek's Eyes page.

Light travels in a straight line unless something gets in the way. When light comes in contact with matter, its travel can be affected. Matter can be classified as transparent, translucent, and opaque.

A transparent substance allows light to pass with only a little interference. Air, windows, clear water, and plastic wrap are considered to be transparent.

A translucent substance allows light to pass through, but it scatters it just enough that images on the other side cannot be seen clearly. Shower doors, wax paper, and plastic milk jugs are often translucent.

An opaque substance allows no light to travel through. It stops the light altogether. Doors, walls, chairs, concrete, and wood are all opaque.

When an opaque object gets in the way of a light beam, light rays are stopped from traveling and an area of darkness appears in the shape of the object. The dark area is known as a shadow. Because your body is opaque, when you stand with the sun behind you, you will see your own shadow. The size and shape of the shadow depend on the position of the light source and its distance from the object. You may have noticed that when the sun is high in the sky in the middle of the day, your shadow is quite short. Later in the day when the sun is lower, your shadow is longer. People have been telling the time using shadows for thousands of years.

Lasers are special forms of light that are used to perform surgery, cut through metal, show movies, play music, read bar codes, make copies and so much more. But what is a laser? A laser is a narrow beam of light composed of light waves of similar wavelengths. The light is specially focused so that it is concentrated to a pinpoint. Because a laser light is so focused and does not spread out much, it can be used for very precise tasks. Learn more about lasers from NASA.

The science of light is known as optics. Scientists and engineers investigate how light behaves and interacts with matter, and develop technologies that make the world better. A digital camera, a wireless mouse, and a Blu-ray disc are all made possible by the science of optics. The properties of light are used to explore the universe, improve people’s health, and care for the environment. 

Are you interested in learning about careers in optics? Learn more about the science of optics and its applications in medicine, astronomy, and engineering.