Lighting a Match with a Laser

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Finding a Green Laser

Finding this laser on amazon is a little tricky because the listings usually only last about a week before being taken down (I suspect it's because it is too powerful to be sold online like this). So instead of a link, I would recommend just to search for the following description:

"​Long Range High Power Tactical Flashlight"

There are a few different options that show up but I can usually find one that is USB powered and under $15 :)

Warning - Be VERY CAREFUL with these lasers. It has been shown in studies like this one that these cheap green lasers work by frequency doubling infrared light and that some lasers have a significant amount of "leaking" invisible but dangerous IR. This is a demo that should be done only by the teacher in a controlled space with laser safety glasses if possible.

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Setting up the Laser

The laser that I use comes with a couple endcaps that need to be removed or modified before lighting a match.​

Diffraction Grating - Many lasers come with a cool diffraction grating that transform the beam into hundreds of dancing points of light. While this looks sweet, you can take this filter off.

Lens - the second adjustable end cap is a lens that sends the beam in a perfect parallel path to extend the range of the laser. If you unscrew this all the way, it focuses the light beam to a point at around 6-8 cm from the lens. After this point, the beam diverges so the pattern on a distant wall actually looks much wider than it did before

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Lighting the Match

I have found that the choice of matches matters a lot in making this demonstration successful. Here are the two characteristics that all of the matches have had in common when I've gotten this to work:

• Matchbook style - The flimsy cardboard matches seem much easier to ignite with a laser than sturdy wooden matches
• Dark tip - We have some matchbooks in the back room that have white tips but they are way too reflective to absorb enough energy and ignite. The classic red-tipped matches seem to work well though.

Once you have the right match, all you should have to do is aim the laser at the match tip and position it right at the focal point. This point is usually around 6-8 cm but you will have to experiment until you find the sweet spot where the match starts to send off whisps of smoke.

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Extensions

Honestly, the laser energy isn't quite as powerful as students are thinking, you can't feel it get hot if you put your hand in the focal point or anything like that but there are still some other things that you can do to demonstrate the energy beyond lighting a match.

One of my favorite extensions is to use the laser to light a balloon. You will still need to focus the laser and hold the balloon at the focal length like you did with the match, but there is one additional step that is needed as well. Since you want the laser's energy to be absorbed by the rubber and not enter into the balloon, you will need to make the balloon's surface dark and opaque. I have found that simply coloring a spot on the balloon with a black sharpie usually does the trick. Once that is done, aim the focused laser at the dark spot and hopefully you can get that balloon to pop! :)

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Troubleshooting

Sometimes it feels like everything was set up correctly but it doesn't seem to be working... Here are the things that I typically try while troubleshooting this demonstration:

• Make sure that the laser is focused by unscrewing the lens cap until it stops
• Try other focal distances. It needs to be right on and a centimeter or two in either direction can really make a difference
• Try different matches. It's possible that they are damp or that the brand that you are trying doesn't ignite as readily
• Recharge the laser. I've noticed that this works best right after the laser has been fully charged. After being used for a while, I've sometimes seen that the beam is working just fine but doesn't have enough power to ignite the match.
• Purchase a new laser. I had one laser that after a year wouldn't light a match any more no matter what I did. My guess is that there was something wrong with how it was focusing or that it could hold a full charge anymore. I fixed this by just buying another $13 green laser and now I use the old one for refraction demos.

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