Why did a blue laser make my nuts glow?

13 March 2011


Dear Chris,

A while ago I ordered some blue lasers online, I got them a few weeks later and was having a blast lasing things around the house to see if they do anything strange, like fluoresce or something, I happened to hit a bag of cashews with the laser and i noticed something very strange, the cashews actually showed some phosphorescence, they glowed a orangy green for about a second after I stopped shining the laser on them, they were actually showing weak BUT VISIBLE phosphorescent properties, I was very excited and started trying other organic things with the laser, I also found out that cashews, peanuts, and almonds all had the same effect to a limited degree, it turns out that peanuts has the longest and brightest glow of the different nuts I tested, also wasabi glows but very poorly. then i tested peanut butter and low and behold it glowed as well and better than all the others better than plain peanuts, (skippy brand) then I went on the internet and didn't find even a hint of a reason why this happens.

If you are stumped do i get a prize? 


Matthew Katzenstein (Summit, NJ, USA)


Chris - There was a paper that was published by some researchers in Athens about 7 or 8 years ago and they were looking at things like olive oil. They also looked at some other nut oils. Their motivation for looking at the fluorescent property of oils was because they wanted to try to come up with a test in order to prove that a sample of oil was authentic, because lots of people sell virgin olive oil for example, and often, it's a mixture of horrible old stuff with a few other bits and pieces to make it look nice and green, and vibrant as though it is [the real stuff].

So, they wanted to know if they were to shine light into the sample, would it interact with the light in a certain way, in order to produce a fingerprint fluorescence pattern or something that they could use as a marker. And they were really surprised to find, yes, it does, and so, I think what's going on with your glowing nuts is that when you shine the blue laser light in, there are molecules in the nuts, specifically, there'll be oil molecules which absorb the energy in the blue light, and they then re-radiate or re-emit the energy shifted towards the green end of the spectrum. That's what these people found in Athens, that these oils soak up the energy - and especially vitamin E - soak up energy in the blue end of the spectrum, and re-emits it in the green. So it soaks up energy at about 450 nanometres bluish and re-emits at about 525-530 nanometres that's green. As a result, you get this nice colour glowing coming through. So I think that's probably what you're seeing going on. Sarah...

Sarah - Does that mean that if you used a red laser, you wouldn't get the same effect?

Chris - Probably, because the molecules are sensitive to the wavelength of blue light and that's how the energy is inputted in the first place. But then after the laser is turned off, there is enough energy still in the molecule that it then, when it relaxes back to its original unexcited state, re-emits the energy at the green wavelength, and that's what happens

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