Dot Physics

Green Laser and Fluorescence

Do you have a green laser pointer? You should. They are relatively inexpensive now. What if you take your laser pointer and aim it at stuff in your house? The laser dot always looks green, right? This is actually a pretty important point about color. Suppose I have the lights on in a room and I look at a blue book. White light is basically all the colors of the visible spectrum, so a diagram like this might be appropriate:

i-24ebe0105f470cd87d7303bb47fd121c-2010-02-11_untitled_3.jpg

So, the book looks blue (since that is what your eye sees). Note that I just showed blue light coming off the book, really the other colors reflect some too but just not as much. What if I shine a green laser at the blue book? Maybe this would be an appropriate diagram:

i-52b490aa3748793f21d05786d69a0bf2-2010-02-11_untitled_4.jpg

So, try this out. Shine your green laser pointer. Only green light is hitting stuff, so only green light can reflect.

Are you ready? Check out this video I made with my green laser pointer.

Crazy – isn’t it? Here is another video. Same thing, but I put one of these cheapo spectral glasses in front of the camera. This splits the light into different colors. You can see that when the laser is pointing on most things, only green light is reflecting off. However, on some objects (orange plastic and pink plastic) other colors are present also.

Just a note: I first noticed this when I was playing with my green laser pointer during Halloween. I randomly pointed the laser at one of those orange plastic pumpkin thingys and it didn’t look green. I thought I was crazy or maybe it was some type of optical illusion. Then a colleague also noticed the same thing – it was then I knew I wasn’t crazy (well, I still might be crazy).

So, what is going on here? This is called fluorescence. The basic idea is that the green light is exciting the electrons in the material to a higher energy level. These electrons then make a transition back down to ground state through a different transition. When light and matter interact, it can be confusing. The main point is that there is a correlation between the change in energy of the electron levels and the frequency of light – for both absorption and emission. The higher the frequency of light, the larger the possible change in energy levels. This is why a green laser is important. A red laser has a lower frequency, so if it did cause some material to fluoresce it might produce light at a lower frequency than red – which your eye is not very good at detecting.

Comments

  1. #1 Fran
    February 11, 2010

    Yes!
    One of our local physics teachers demo’d this at our Fall Demo Night a few years ago. I bought a green laser pointer just for that! One of my colleagues has bought a BLUE laser pointer…now I have laser envy.

  2. #2 Rhett Allain
    February 11, 2010

    @Fran,

    I totally want a blue laser pointer. It looks like the prices on those are coming down – so maybe I will have one soon. Then I will be unstoppable – right?

  3. #3 Joseph
    February 11, 2010

    If only I weren’t a postdoc and actually had some disposable income!
    http://onpointlasers.com/presentation-laser-pointers/blue-violet-laser-pointers.html

  4. #4 Siamang
    February 11, 2010

    Thank you for FINALLY making me understand what florescence is!

  5. #5 NJ
    February 11, 2010

    Of course the only real reason to have a green laser pointer is to illustrate how the diffraction maxima change positions when you alternate aiming red and green laser beams at an appropriate grating!

  6. #6 Siamang
    February 11, 2010

    And THAT’S why florescent lights are violet or ultra-violet. I get it!

  7. #7 Dave
    February 11, 2010

    The same effect (fluorescence) also occurs with ultraviolet LEDs. However, since the human eye is very insensitive to ultraviolet light, the effect is even more pronounced. Plus, UV LEDs are quite inexpensive (much cheaper than laser pointers).

    An additional trick you can use is to wear a pair of spectrally sensitive glasses that block the light from the LED/laser diode. This allows you to see much weaker fluorescence without the eye being overloaded by the primary colour.

    Also, note that fluorescence is almost always a longer wavelength, unless you happen to be playing with some of those frequency doubling crystals (KDP?).

    Another interesting effect is phosphorescence, which UV LEDs will cause.

    Now, who wants to explain phosphorescence? ;-)

    Dave

  8. #8 rob
    February 11, 2010

    a yellow laser. *that* would be cool.

  9. #9 Anonymous Coward
    February 11, 2010

    Re: #7

    To nitpick,

    Frequency upconversion in KDP (like what is going on inside Rhett’s laser pointer to make green light from the infrared laser running inside the thing) is usually not called fluorescence.

    Fluorescence generally refers to single-photon effects (response linear in intensity) like Rhett’s example. Frequency doubling in KDP is a two-photon effect (response nonlinear in intensity).

  10. #10 Anonymous Coward
    February 11, 2010

    Re: #8

    Yellow lasers exist. Ferinstance, a dye laser can be made to produce yellow light.

    I also thought they were quite cool when I first saw them (especially since if you open the cavity up you can see the intense intracavity beam going through the air just like in the movies).

    But after getting sprayed with laser dye a few times (and having to continually fight with the damn things to get them to work) I’ve changed my opinion.

  11. #11 Jospeh
    February 11, 2010

    Yellow/Orange research lasers:
    http://onpointlasers.com/research-oem-lasers/yellow-orange.html

    Red, Yellow, and Green all-in-one (I only see one button, so not sure how it works)
    http://onpointlasers.com/fun-lasers/triple-threat-red-yellow-and-green-in-one-laser-pointer.html

    I swear they used to have an orange laser pointer, but now I don’t see it anymore. (this is perhaps leaving me open for a laser-in-eye comment ;)

  12. #12 Viadd
    February 12, 2010

    Royal-violet lasers (405 nm, so a bit off the eye’s sensitivity peak com compared to ‘blue-ray colored blue lasers). $20 for a laser pointer, twice that for danger of blinding not shippable to the US. They also sell cheap greens. (And lots of other cheap Chinese crap.)

    http://www.dealextreme.com/products.dx/category.923

  13. #13 Jon H
    February 12, 2010

    “I totally want a blue laser pointer. It looks like the prices on those are coming down – so maybe I will have one soon. Then I will be unstoppable – right?”

    I’m about to order a $12000 500mW blue laser for the lab I work in. Neener neener.

  14. #14 CCPhysicist
    February 13, 2010

    Could you repeat this experiment with a good quality diffraction grating instead of those stupid glasses? The spectrum might be interesting.

    Also just a reminder to readers that if you have one of those more expensive green laser pointers, the ones used to point out stars in the sky, looking at its scattered light can be dangerous. Be sure you know what “class” it is and what precautions to take.

  15. #15 MRW
    February 13, 2010

    Re #9

    Single-photon is not the distinction. Two-photon fluorescence exists and is also different from frequency-doubling.

    You’re right, though, that what happens in a KDP crystal is not fluorescence.

  16. #16 David Walley
    April 19, 2010

    the red emission seen from impinging the 532nm laser on the halloween plastic is coming from rxn with Rare earth doped strontium aluminate blended into the plastic. or similar material..basically a Stokes conversion…Cornbread photonics lab in Mississippi

  17. #17 danielpowerha
    July 5, 2010

    That’s a cool experiment,I am going to buy a laser pointer from http://www.jetlasers.org and do the experiment myself.

  18. #18 amreen
    September 2, 2010

    please suggest some projects regarding this

  19. #19 joe kess
    May 18, 2011

    Try shingin a green laser pointer through light (clear) corn syrup.

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