13.3: Light and Color

Figure 13.3.1

This rainbow contains all the colors that you can see in the land below it—the yellow of the hills, the green of the leaves and grasses. It contains other colors as well. In fact, a rainbow contains all of the colors of visible light.

Wavelength and Color

Visible light is light that has wavelengths that can be detected by the human eye. The wavelength of visible light determines the color that the light appears. As you can see in the Figure below, light with the longest wavelength appears red, and light with the shortest wavelength appears violet. In between are all the other colors of light that we can see. Only seven main colors of light are actually represented in the diagram.

Separating Colors of Light

A prism, like the one in the Figure below, can be used to separate visible light into its different colors. A prism is a pyramid-shaped object made of transparent matter, usually clear glass or plastic. Matter that is transparent allows light to pass through it. A prism transmits light but slows it down. When light passes from air to the glass of the prism, the change in speed causes the light to change direction and bend. Different wavelengths of light bend at different angles. This makes the beam of light separate into light of different wavelengths. What we see is a rainbow of colors.

Q: Look back at the rainbow that opened this article. Do you see all the different colors of light, from red at the top to violet at the bottom? What causes a rainbow to form? A: Individual raindrops act as tiny prisms. They separate sunlight into its different wavelengths and create a rainbow of colors.

Launch the PLIX Interactive below to observe the refraction of white light through a prism. You can adjust the angle of the white light shining on the prism and observe its effect on how each color of light refracts (or bends):

Colors of Objects

An opaque object is one that doesn’t let light pass through it. Instead, it reflects or absorbs the light that strikes it. Many objects, such as the leaves pictured in the Figure below, reflect just one or a few wavelengths of visible light and absorb the rest. The wavelengths that are reflected determine the color that an object appears to the human eye. For example, the leaves appear green because they reflect green light and absorb light of other wavelengths.

A transparent or translucent material, such as window glass, transmits some or all of the light that strikes it. This means that the light passes through the material rather than being reflected by it. In this case, we see the material because of the transmitted light. Therefore, the wavelength of the transmitted light determines the color that the object appears. Look at the beautiful stained glass windows in the Figure below. The different colors of glass transmit light of different colors.

The color of light that strikes an object may also affect the color that the object appears. For example, if only blue light strikes green leaves, the blue light is absorbed and no light is reflected.

Q: What color do you see if an object absorbs all of the light that strikes it?

A: When all of the light is absorbed, none is reflected, so the object looks black. But black isn’t a color of light. Black is the absence of light.

The Colors We See

The human eye can distinguish only red, green, and blue light. These three colors are called the primary colors of light. All other colors of light can be created by combining the primary colors. Look at the Venn diagram below. Red and green light combine to form yellow light. Red and blue light combine to form magenta light, and blue and green light combine to form cyan light. Yellow, magenta, and cyan are called the secondary colors of light. Look at the center of the diagram, where all three primary colors of light combine. The result is white light.

Have you ever wondered why we see the colors we do? Watch the video below to discover more about the science behind visible colors:

Pigments

Many objects have color because they contain pigments. A pigment is a substance that colors materials by reflecting light of certain wavelengths and absorbing light of other wavelengths. A very common pigment is the dark green pigment called chlorophyll, which is found in plants. Chlorophyll absorbs all but green wavelengths of visible light. Pigments are also found in many manufactured products. They are used to color paints, inks, and dyes. Just three pigments, called primary pigments, can be combined to produce all other colors. The primary colors of pigments are the same as the secondary colors of light: cyan, magenta, and yellow.

Q: A color printer needs just three colors of ink to print all of the colors that we can see. Which colors are they?

A: The three colors of ink in a color printer are the three primary pigment colors: cyan, magenta, and yellow. These three colors can be combined in different ratios to produce all other colors, so they are the only colors needed for full-color printing.

What color would a white car appear under yellow light? What would you see when you look at an orange car through green glasses? Why? Use the sliders in the Rose-Colored Glasses simulation below and play around with setting different colors for the light source, the car, and the glasses to answer these questions and learn more about the light and color:

Summary

• The wavelength of visible light determines the color that the light appears. Light with the longest wavelength appears red, and light with the shortest wavelength appears violet. In between are the wavelengths of all the other colors of light.
• A prism separates visible light into its different colors. As light passes through the prism, it slows and bends, but different wavelengths bend at different angles. This separates light into different wavelengths, forming a rainbow of colors.
• The wavelengths of visible light that an object reflects or transmits determine the color that the object appears to the human eye.
• The human eye can distinguish only red, green, and blue light. These three colors are the primary colors of light. All other colors of light can be created by combining the primary colors. Secondary colors of light—cyan, yellow, and magenta—form when two primary colors combine equally.
• Pigments are substances that color materials by reflecting light of certain wavelengths and absorbing light of other wavelengths. The primary pigment colors are cyan, yellow, and magenta. They can be combined to produce all other colors.

Review

1. What determines the color of visible light?
2. Which color of light has the longest wavelength? Which color has the shortest wavelength?
3. How does a prism separate visible light into its different colors?
4. To a person with normal vision, the apple in the Figure below appears green. Explain why.

5. The human eye can detect only three colors of light. What three colors are they? How can we perceive other colors of light?
6. What are pigments? Identify the primary colors of pigments. If you combined the three primary pigment colors, what color would you get?