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:
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:
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.