READ: Light and Color

1. Light

Slip! Slop! Slap! Did you ever hear this slogan? It stands for slip on a shirt, slop on some sunscreen, and slap on a hat. The slogan originated in Australia in the 1980s, but it has since been adopted in many other places around the globe. It sums up simple steps you can take to protect your skin from sunlight. Sunlight consists of a wide range of electromagnetic waves, some of which are harmful.

The Waves in Sunlight

Electromagnetic waves are waves that carry energy through matter or space as vibrating electric and magnetic fields. Electromagnetic waves have a wide range of wavelengths and frequencies. Sunlight contains the complete range of wavelengths of electromagnetic waves, which is called the electromagnetic spectrum. The Figure below shows all the waves in the spectrum.

Let There Be Light

Light includes infrared light, visible light, and ultraviolet light. As you can see from the diagram above, light falls roughly in the middle of the electromagnetic spectrum. It has shorter wavelengths and higher frequencies than microwaves, but not as short and high as X rays.

Q: Which type of light do you think is harmful to the skin?

A: Waves of light with the highest frequencies have the most energy and are harmful to the skin. Use the electromagnetic spectrum above to find out which of the three types of light have the highest frequencies.

Infrared Light

Light with the longest wavelengths is called infrared light. The term infrared means “below red.” Infrared light is the range of light waves that have longer wavelengths and lower frequencies than red light in the visible range of light waves. The sun gives off infrared light as do flames and living things. You can’t see infrared light waves, but you can feel them as heat. But infrared cameras and night vision goggles can detect infrared light waves and convert them to visible images. For a deeper understanding of infrared light, watch the video at this URL: 

Visible Light

The only light that people can see is called visible light. This light consists of a very narrow range of wavelengths that falls between infrared light and ultraviolet light (See figure below). Within the visible range, we see light of different wavelengths as different colors of light, from red light, which has the longest wavelength, to violet light, which has the shortest wavelength (see the spectrum below). When all of the wavelengths of visible light are combined, as they are in sunlight, visible light appears white. You can learn more about visible light at this URL: 

Ultraviolet Light

Light with wavelengths shorter than visible light is called ultraviolet light. The term ultraviolet means “above violet.” Ultraviolet light is the range of light waves that have shorter wavelengths and higher frequencies than violet light in the visible range of light. With higher frequencies than visible light, ultraviolet light has more energy. It can be used to kill bacteria in food and to sterilize surgical instruments. The human skin also makes vitamin D when it is exposed to ultraviolet light. Vitamin D, in turn, is needed for strong bones and teeth. You can learn more about ultraviolet light and its discovery at this URL:

Too much exposure to ultraviolet light can cause sunburn and skin cancer. As the “slip, slop, slap” slogan suggests, you can protect your skin from ultraviolet light by wearing clothing that covers your skin, applying sunscreen to any exposed areas, and wearing a hat to protect your head from exposure. The SPF, or sun-protection factor, of sunscreen gives a rough idea of how long it protects the skin from sunburn (see figure below). A sunscreen with a higher SPF value protects the skin longer. Sunscreen must be applied liberally and often to be effective, and no sunscreen is completely waterproof. 

Q: You should apply sunscreen even on cloudy days. Can you explain why?

A: Ultraviolet light can travel through clouds, so it can harm unprotected skin even on cloudy days.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/

2. Color

This rainbow contains all the colors that you can see in the field below it—the pink and yellow of the flowers, 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. For an animated version of the prism below, go to this URL:

http://en.wikipedia.org/wiki/File:Light_dispersion_conceptual_waves.gif

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.

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 below 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. You can explore the colors of visible light and how they combine at this URL:

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.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/

3. Summary and Vocabulary

Summary

• Sunlight contains the complete range of wavelengths of electromagnetic waves. The entire range is called the electromagnetic spectrum.
• Electromagnetic waves that are commonly called light fall roughly in the middle of the electromagnetic spectrum. Light includes infrared light, visible light, and ultraviolet light.
• Infrared light is light with the longest wavelengths and lowest frequencies. You can’t see infrared light, but you can feel it as heat. Besides the sun, flames and living things give off infrared light.
• Visible light consists of a very narrow range of wavelengths that falls between infrared light and ultraviolet light. It is the only light that people can see. Different wavelengths of visible light appear as different colors.
• Ultraviolet light has shorter wavelengths and higher frequencies than visible light. Ultraviolet light also has more energy, which makes it useful for killing germs. Too much exposure to ultraviolet light can damage the skin.
• 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.

Vocabulary

• infrared light: Part of the electromagnetic spectrum in which waves have a wavelength between those of radio waves and visible light.
• ultraviolet light: Electromagnetic radiation with a wavelength falling between the wavelengths of visible light and X rays.
• visible light: Range of wavelengths of electromagnetic waves that the human eye can detect.
• pigment: Substance that colors materials by reflecting light of certain wavelengths and absorbing light of all other wavelengths.
• primary color: One of three colors of light (red, green, or blue) that can be combined to produce all other colors of light.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/