Light & Color in the "Wizard of Oz" Room
[Shopping List: Low-pressure Sodium Vapor Lamps; lots of colorful objects such as stuffed animals, photos, candies, fruits, vegetables, balloons, posters, photographs, etc.]
- Cover any windows with cardboard to completely darken a small room.
- Fill the room with lots of very colorful objects (avoid fluorescent or "neon" materials- see notes below)
- Turn off the main incandesant or fluorescent lights, and illuminate the room with only one or two low-pressure sodium (LPS) vapor lamps. Notice that everything in the room appears to be kind of sepia-toned blach & white, or shades of monochromatic yellow and black. There are no other colors visible in the room.
- Turn on the main overhead lights (leave the LPS vapor lamps on) and all the colors in the room suddenly appear.
- Turn the main overhead lights out again, and use only a few small flashlights in various colors to "explore" the room.
What's Happening: We've all heard the phrase "If a tree falls in the forest and no one is there to hear it, does it make a sound?" In this room we pose a similar situation: if a room full of multi-colored objects is illuminated with only a single color of light, what color are the objects really? Is an apple still red, a carrot still orange, etc? The answer is that objects do not "have" colors, but we can measure the colors in the light that is coming from those objects. We detect or measure red if and only if there is at least some red light coming from that object. The same is true of any other color. Color is to light what tone or note is to sound. Each individual color of light corresponds to a different frequency or wavelength of electromagnetic radiation within what we call the visible spectrum, just as each individual tone or note on the musical scale corresponds to a different frequency or wavelength of sound.
In this room the only ilumination is a sodium vapor lamp which emits a pure yellow wavelength or frequency of light- no other color exists. And since yellow is the only color present, it is the only color we will see, regardless of the "colors" of the objects in the room. Everything will appear to be some shade of yellow or black, depending on how much of the yellow light from the lamps is reflected back into our eyes. This is sort of like living in a black & white movie, or when Dorothy was in Kansas at the beginning of "The Wizard of Oz."
In most cases, the light (and thus color) we see is reflected from an object into our eyes (as opposed to objects which produce light, such as a flame, a glowing metal wire, a light stick, or a fluorescent material). If an object does not produce its own light, then we must use another source of light to illuminate it, such as the sun, or, as in this case, a light bulb. Light from that source hits the object, where some of it is absorbed by the (based on the composition of the object- i.e. what it's made of). Some of it may also pass through the object. Any light which is not absorbed or or transmitted through the object is reflected, and if we like, we can measure its intensity. If the source light contains many different colors, then individual colors may be absorbed, transmitted or reflected by different amounts, again depending on the compsition of the object in question. The intensity or amount of each color can be measured, but we can never detect or measure any color which was not present in the original source of illumination. If the illumination source does not contain any red light, we will never detect any red in the reflected light. Even if the source light does contain red, the object may absorb all of the red light, in which case we would also measure no red light reflected. In this case, the object will appear dark or black.
The main lights in the room, whether they are incandesant or fluorescent, are designed to emit nearly all of the colors in the visible spectrum, rather than just the one yellow wavelength of the LPS lamp. When this happens the light appears white. "Pure" white is not really a single color, but rather the combination of all the colors our eyes can see. Thus when we turn on this "white" light source all colors of light will strike each object in the room, and for each object those colors which are not absorbed can be reflected back into our eyes to produce the colors we expect. It's as if we are suddenly in the colorful land of "Oz."
This also works when we use a small white flashlight to illuminate only a portion of the room. Objects within the white beam will appear "in color." On the other hand, if we use a red flashlight, objects in the beam can only appear red, black or some shade in between, but no other color. If we combine flashlights of different colors we can see new colors appear in the combined beams.
Things can get even more complicated. Just as several different musical notes can be combined to produce a complex sound we call a chord, because of the way our eyes work, different colors of light can be combined to appear to us as new colors, even if this new color itself is not present. For example, if a pure green beam of light and a pure red beam of light are combined into a single beam or spot, this beam or spot will appear yellow, even though there is actually no light corresponding to the frequency or wavelength of yellow. This "yellow" light is, in a sense, an illusion created only by our eyes (or brain if you prefer). This is an example of additive color mixing, and is not due to the light itself, but rather the way our eyes sense colors of light. This is discussed more in our "Colored Shadows" activity.
Variation: Add one or more fluorescent (or "neon") materials to the room. What colors do you see from these objects? Learn more about what's happening with these objects in our "Phosphorescence: Writing with Light" activity.
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