Light as Energy – Fluorescence and the Photoelectric Effect
Fluorescence is a phenomenon that arises out of the interaction between certain materials and certain frequencies, or energies of light. It is one of the most visually stunning light-matter interactions observable. Who isn’t amazed when invisible light suddenly becomes visible, or when green light turns into red? This very simple and inexpensive hands-on, inquiry activity brings the science of fluorescence to light – and qualitatively demonstrates the photoelectric effect.
Grades: 9 to 12+
Duration: 1/2 Hour – 1 Hour
NGSS Connections PS4.B: Electromagnetic Radiation
- Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter. (HS-PS4-4)
- Photoelectric materials emit electrons when they absorb light of a high enough frequency (HS-PS4-5)
NGSS Connections PS3: Energy
- Energy may take different forms (e.g. energy in fields, thermal energy, energy of motion) (MS-PS3-5)
- The transfer of energy can be tracked as energy flows through a deigned or natural system (MS-PS3-3)
Supplies: Fluorescent markers (yellow, orange and pink), rainbow glasses, and a set of LASER Blox, red, green and violet.
Light as Energy: Background
Fluorescence is observed when light of sufficient frequency (higher frequency = higher energy) is able to “knock” electrons in a material’s molecules into a higher energy state. When the “knocked” electron returns to its stable, original, energy state, it releases energy in the form of light. Because this light is of lower frequency, and longer wavelength, than the originating light, it shows up as visible, or as a different color from the incident light!
I thought long and hard about how to show you a great demonstration of light as energy and fluorescence – a black light in a dark room reveals many beautiful colors when shined at fluorescing objects – but I wanted something that would provide some insight into the science of it all. And, I don’t want you to leave thinking that only “black” or ultra violet light can cause fluorescence. Remember – what is required for fluorescence to occur is light of sufficient energy – not “black” or ultraviolet light. So, I chose a demonstration that would use laser light (I love lasers) of three different wavelengths – a 635nm Red LASER Blox, a 532nm Green LASER Blox and a 405nm Violet LASER Blox – all in the visible spectrum and super easy to use, to demonstrate.
Diffraction of White Light
Now, put on the rainbow glasses, which are made of diffraction film and look around the room at the white lights. You can see that they separate the white lights in the room into beautiful rainbows – AKA, their constituent wavelengths. Very cool. Very pretty.
Diffraction of Red and Green Laser Light
Leave the glasses on, and point the red laser at a plain white wall or piece of paper and what do you see??DOTS, lots of dots! Those dots are the diffraction pattern of the red laser. Repeat with the green laser and you see the same thing.
Because LASER light is monochromatic, there is only one wavelength (for all practical purposes), and therefore, no spectrum, just a diffraction pattern. The monochromatic beam is “split” into a diffraction pattern. See more on the monochromatic nature of laser light HERE
Diffraction of 405nm Violet Laser Light
Now, for something completely different – try it with the 405nm violet laser. What? Why is this monochromatic laser producing a spectrum?? Because even though the 405nm violet laser is in the visible spectrum, it has a very short wavelength, a high enough frequency and sufficient energy to cause LOTS of things to fluoresce, including a plain piece of white paper. Some other things to try are detergents, olive oil and petroleum jelly – as well as any tonic water containing quinine.
What you see with the diffraction glasses is the emission spectrum caused by fluorescence. When the electrons in the paper absorb the 405nm violet light, they become excited and are “knocked” up to a higher energy level. They can not stay up there for long, (much, much less than a second) and quickly fall back to their original energy state. As they do this, they release the extra energy in the form of light of various wavelengths. So, you see a spectrum.
Light as Energy and Fluorescent Materials
What happens when we shine the lasers on “FLUORESCENT” materials, like the markers? Color in three squares, one of each color, and we’ll see what happens!
What do you think will happen with the 405nm violet laser? Will it cause the fluorescent colors to fluoresce? You bet! Notice the different emission spectrum colors. Fluorescent materials are more easily excited.
The green laser has a wavelength of 532nm, longer than a 405nm violet laser, but shorter than red. We know that a 532nm green laser will not cause fluorescence with your plain piece of paper, but what about fluorescent markers? Let’s see.
It does not seem to fluoresce with the yellow fluorescent marker. But it does with both the pink and orange fluorescent markers! What does this tell you about the relative fluorescent-ness (??!) of yellow versus pink or orange? If you shine the 532nm green laser around the room (carefully!) you will notice that many orange and pink objects, even ones you would not necessarily think of as fluorescent, will produce a fluorescence emission spectrum.
Finally – try with the 635nm red laser. This is the longest wavelength, with the least amount of energy. Does it cause any of your fluorescent squares to fluoresce. No. It does not. 635nm light is not energetic enough to cause fluorescence with our materials.
This is also a great (poor man’s) way to qualitatively demonstrate the photoelectric effect!