Experiment #3 Color Absorption (matches)

Introduction

Who knew that lasers were useful to demonstrate some of the most fascinating properties of the universe? Well, the keen staff at NOVAlasers did! And, of course, we’re always excited to share our knowledge!

Look around you. You’re surrounded by color. So much so, that most of us stop thinking about it! The human brain tends to simplify things for us and unless we pause and think, color sometimes seems like an intrinsic property of an object itself. Color, though, is a combination of the properties of an object, the light source striking that object, and the sensory information received by our eyes and interpreted by our brain. In short, the reason that an object is colored red, for example, is that when white light (sunlight or light from a light bulb) strikes that object only the red wavelength is reflected back into our eyes. The rest of the wavelengths are absorbed by the object. The reason that an object appears white is because it reflects all of the wavelengths back into our eyes, and absorbs none. Our eyes then interpret the combination of wavelengths reflected back as the color white.

So color then, is not really an intrinsic property of an object itself, but a combination of the properties of an object and the sensory perception of our eyes and brain. We learn to associate color names such as white, blue, etc early on in life because others teach these color names to us. The sky is blue and the grass is green. But who’s to say what each individual person (each individual pair of eyes and brain) actually perceive? While you and I both call the sky blue we could actually be perceiving two totally different things! If you borrowed my brain you could be shocked to see that what I call blue is what you’ve learned to call green! Philosophers call this color qualia. But since we’re laser physicists and not philosophers let’s move on to some experimentation!

With the aid of your green laser and some colored objects you can very easily demonstrate to yourself and others the nature of color.

Background

• Laser light is near-monochromatic (consisting of a narrow band of wavelengths) and so is of one color. A green laser produces green light (532nm wavelength).

• Color is not an intrinsic property of an object but a combination of the properties of an object and the way your eyes and brain receive and interpret sensory data. The reason that an object appears blue is that it reflects the blue wavelength only while absorbing other wavelengths.

• On a hot, sunny summer day is it better to be wearing a black t-shirt or a white t-shirt? A white t-shirt would be the better choice as it would keep you cooler. This is so because white reflects all wavelengths while black absorbs all wavelengths. The light energy is then converted into heat energy upon impact with the black t-shirt.

Purpose: to observe the nature of color with the aid of a laser.

Materials

1. High powered green laser (at least 100mW)
2. White-tipped matches and red-tipped matches
3. Black marker
4. Laser safety goggles
5. Stop watch
6. Laser mounting apparatus
7. Match mounting apparatus
8. Proper container to discard used matches

Safety: in this experiment >5mW (ClassIIIb) green laser light is required. NEVER point the laser at yours or anyone else’s eyes and be mindful of the laser light reflecting off of surfaces. Laser safety goggles are highly recommended. Observe all fire safety.

Procedure

1. Mount a match in your match mounting apparatus (can be as simple as sticking the match into a chunk of clay)
2. Mount your laser in a mounting apparatus, place it on a stack of books, or hold it in your hand. Mounting the laser will produce the most accurate results.
3. Mark the placement of the laser pointer and the match as this placement will need to be duplicated as closely as possible for each match you light. The recommended distance between the laser and the match is about 1-2 inches.
4. Hold the stop watch in a free hand.
5. Turn the laser on and direct it at the match. At the same time start the stop watch.
6. When the match has ignited immediately stop the stop watch.
7. Record the time it took for the laser to ignite the match. If the match does not ignite after 60 seconds record it as a failure to ignite the match.
8. Repeat the process for both the white-tipped match and the red-tipped match. Also, use the black marker to color the white-tipped match black and repeat the experiment.
9. Do as many repetitions of the time measurement as you wish.

Observations

Review the observations you have made.

Explanation

Red-tipped matches and white-tipped matches colored black by the black marker: these will ignite in the shortest amount of time as they both absorb green light.

White-tipped matches: these will take much longer to ignite, usually not igniting at all, as they reflect green light.

Conclusion

Objects are said to have a particular color based on which wavelengths of light they reflect and which wavelengths they absorb and how this information is processed by our eyes and brain.

Copyright 2007 NOVAlasers INC