(LSEL) The Electromagnetic Spectrum Lesson

The Electromagnetic Spectrum

The electromagnetic spectrum includes a wide range of light waves, some we can see and others we cannot see. Some of the non-visible types of waves are radio waves, microwaves, infrared rays, and X-rays. These types of waves have all sorts of uses in science and technology. We will discuss these uses and see how it impacts our lives later in the module. Now we are going to concentrate on the visible part of the spectrum. We will focus on the properties that allow us to see various colors and why certain wavelengths are visible.

Visible light is the narrow range of photons that move at a wavelength we can see. Among the different photons that are in visible light, the ones having the longest wavelength look red to us. The ones with the shortest wavelength look blue to us. All other colors come from light with wavelengths that fall between the red and blue range. When we see an object, what we are seeing is reflected light. When light hits an object, some wavelengths are absorbed by the object and some are reflected. When we see an object of a certain color, it means the light of that color's wavelength is being reflected off the object. When light of different wavelengths enter our eye, it is converted to an electrical impulse our brain is able to perceive as color. For example, when you see a red shirt, the shirt is absorbing all colors of light except for the red wavelength.

Black and white images are a bit different from colored images. White is a combination of all colors.  When we see white, the object is reflecting all wavelengths of color. Black is the opposite of white.  It is the absence of all light. When we see a black object, all colors are being absorbed and none are reflected back to our eye. We can see that white light contains the entire visible spectrum of colors when we see a rainbow. A rainbow forms when white light from the sun is split by raindrops acting as a prism and a mirror. A prism is a substance that can refract light, splitting white light into the colors of the visible spectrum. In a rainbow, both refraction and reflection occur so the light is separated into colors and reflected back to your eye. As sunlight travels through a raindrop, it is refracted into the colors of the rainbow. When the light hits the back of the raindrop, it is reflected back as the colors of the rainbow you see.

Additive Colors

Light from the three additive primary colors may be combined to make any other color. These three additive colors are red, blue, and green. Combining the wavelengths of the three primary colors leads to the perception of a wide array of colors. Our eyes are able to see these three colors and determine how much of each wavelength is reaching the eye. Our brain is then able to process this information and determine which color we are actually seeing. We will see in the next lesson how this same principle is used to create images on a computer and television screens.

***You may be confused about the primary colors at this point. Remember that the primary colors you learned in art class: red, yellow, and blue, are for pigments. The primary colors here: red, green, and blue, are for light.***

Subtractive Colors

When we see white light we can subtract colors to get any other color, as well. Your eyes use the primary subtractive colors to filter or remove light of certain colors. The primary subtractive colors are cyan, magenta, and yellow. These colors can take away certain wavelengths by absorption instead of reflecting them. This prevents those wavelengths from reaching the eye so that we do not see those colors. In this way colors are subtracted from the primary colors and this changes our perception of the color that we are viewing.

Light and the Atmosphere

Most of the light on Earth comes to us from the Sun. The Sun emits billions of photons every second, and because the sun is a sphere these photons are sent out in all directions. The photons that reach Earth are the few that are sent out in our direction. When these photons reach the Earth, they first run into Earth's atmosphere. Some of the photons get absorbed by the atmosphere itself, especially the ultraviolet wavelengths whose wavelength is shorter. The layer that absorbs these ultraviolet photons is called the ozone layer. This is a good thing for us, because too much ultraviolet light would kill us, and all other living things on the planet. Ultraviolet light is also responsible for sunburn and skin cancer. Some of man's activities over the past century have destroyed some of the ozone layer. This has increased the amount of ultraviolet radiation that reaches the Earth. The effects of increased ultraviolet radiation are being studied and it remains to be seen how severe the effects will be over time.

Other wavelengths of light, such as the ones responsible for blue sky and blue water, are not absorbed. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue light is scattered more than other colors because it travels shorter wavelengths. This is why we see a blue sky when the sun is high above the horizon. Most of the other wavelengths of visible light make it through the atmosphere and reach the surface of the Earth. This is why we are able to see the many colors contained in white light that we do.

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