Science Buddies: "Ask an Expert"

Hello everyone, I'm testing to see what type of material works best as a bike reflector (which material reflects the most light). I need help in making a list of which materials to test, preferably cheap and easy to wrap around a bike reflector (ex: foil), also the location as to where to get the material.

Also, I'm planning on doing the experiment by putting the different materials in a pitch-black environment and shining a flashlight at different angles and distances. Is this the best way to go about? Also, is there a device that's relatively cheap that can measure the amount of light reflected?

Thanks for any help.

Dear Rocket Man,

Welcome to the wonderful world of photometry.

This project takes me back to one of the science fair projects that I conducted in the 6th grade. I too needed a photometer. The one I built was very crude, using a cardboard tube from a roll of paper towels, and a solar cell as the light detector. To measure the light, I used an earphone to hear the loudness of the clicks that occured as I opened and closed a switch in the simple series circut.

Obviously, this was a subjective measurment as well... but it served to get me first place. However, for you, a little more repeatability, with numbers is needed.

I suggest that you look into simple photometers and how they are made. (here is the obligatory suggestion of performing a web search)...

You can make a more accurate and repeatable photometer, the same way I did in the 7th grade... by replacing the switch and earphone arrangement with a low cost voltmeter. You can buy a solar cell and a pocket voltmeter or multimeter at your local electronics hobby store. Alternatively, you can buy a photodiode and use the diode/transistor testing setting of the ohmeter on your pocket multimeter. The photodiode works as a miniture solar cell... pushing electricity through the diode when it is reverse biased (another websearch is suggested here) Placing a double convex lens in front of the photodiode will allow you to concentrate, focus the light onto the photodiode to make the system more sensitive. Focusing the system is as simple as adjusting the distance such that a sharply focused image of a light source lands on the photodiode.

May I suggest that you perform another websearch using the keyword, "retroreflector"? Retroreflectors are designed to subtantially reflect incident light back to their source. The best known device is called a "corner cube", in which light bounces off of three mirror surfaces placed at right angles to each other. The reflection from each ends up being exactly 180 degrees after bouncing off all three surfaces. Another retroreflector system uses a simple lens, or arrangement of lenses, to focus light onto a reflective surface... since the light from distant sources is focused down to a small point, that small point, when seen as a light source travels the exact same path in reverse, sending the light back to the original distance source.

The first system, the corner cube, is how must bike reflectors made of solid plastic operate, having bounced the light inside of the plastic by "Total Internal Reflection" (another keyword to searh). Look closely at the bike reflector, yhou will see the little corner cubes.

The second system is how most reflector tape and road signs operate. Thousands of tiny glass beads serve as lenses to focus the light on the aluminum foil on the back of the tape or road sign.

For fun, you might try making some corner cubes out of mirrors. You can buy small mirror tiles at a home improvement store. Or, you can try building a lens based retroflector using almost any convex lens such as a magnifying glass.

By the way... eyeballs work as retroreflectors... ever seen the "red-eye" effect in your flash illuminated photographs? Or seen a cat's eyes glowing green in the dark when you shine a flashlight at them? That is the effect of the lens based retroreflector.

Good luck and have fun!

Thanks candicebrownelliott2 can you possibly tell me what materials would be good to use?

What kind of materials would you like to test? What's already on your list?

Have you performed a websearch on reflectivity? More keywords: Lambertian vs. specular reflection

Materials:

Aluminum
Silver
Chromium Over Nickel (so called "chrome metal, like on cars)
Titanium Oxide
Zinc Oxide
White paint (usually Titanium Oxide powder)

Well I doubt that's very useful since I can't go up to a store and say "Can I have some Aluminum Oxide?", so any ideas for more easy-to-get materials? Thanks.

Actually, you can.

Alumina (aluminum oxide) is the incredient in plastic polish, available over the web or at TAP Plastics.

Titanium oxide is found in plain white paint.

Aluminum power is found in "silver" modeling paints. Both of these are available at hobby stores.

copper powder is found in "copper" paint, found at most paint stores for home decorating.

Zinc oxide is found in the white sun block for noses... This is an "old fashioned" material... but you should still be able to find it.

candicebrownelliott2, can you please tell me how to separate the materials from the common items? (such as getting titanium oxide from plain white paint)

Why would you want to separate them? The pigment IS the main optical component of the paint... and is available for this type of experiment.