In this experiment we will see how it is possible to make colors glow and emit light to the surroundings and we will discover what factors affect this glow. It is recommended to have adult supervision during the experiment.
Equipment
Colored markers for drawing; yellow and green give the best results
Transparent glass cups
Water
Device for killing mosquitoes with ultraviolet light
The Experiment
The course of the experiment can be seen in the video
Note: Be careful that water does not splash on the mosquito killer
Explanation
The light and colors that humans can see are a type of electromagnetic radiation. Humans can see only a very small range of all existing electromagnetic radiation. Electromagnetic radiation moves through space like a wave and we can only detect certain frequencies (number of wave peaks per second).
Around us there is electromagnetic radiation at many frequencies, including radio waves, microwave radiation, infrared radiation, ultraviolet radiation, X-rays, and gamma rays.
As mentioned, we can see only a narrow range of frequencies between infrared and ultraviolet, called visible light. When we see something colorful in the world it usually happens because it is illuminated by “white” light that contains all colors and all wavelengths, like sunlight or light from a lamp. The object absorbs some colors and reflects the colors it does not absorb. A material that is red, for example, absorbs all other colors (green, blue, yellow, purple, etc.) and reflects only red.
But a material has another way to be colorful besides simple absorption and reflection called “fluorescence.” In this process a material absorbs light of a certain color (more precisely, the electrons of the material absorb the light). The absorbed light brings the material to an “excited” energetic state. Its electrons move to another state (intermediate state), semi-stable, and after a very short time the electrons return to the original stable state, emitting light in the process.
Because the material itself emits light it appears different from ordinary colored material, as if it “glows” and emits light by itself. This is clearly seen in our experiment, where the color really appears luminous.
Due to these energy transitions to high, intermediate, and low states, the light emitted in the fluorescence process has a slightly longer wavelength (less energetic) than the absorbed light. In our experiment ultraviolet light, which the human eye cannot see, is absorbed and a different wavelength, which the eye can see, is emitted.
The mosquito killer lamp also emits some visible purple light. But there are lamps that emit only ultraviolet light and they are invisible (hence called “black light”). Only if a fluorescent material is placed nearby can we detect that it is indeed emitting light.
Other lamps used in household lighting, as seen in the video, do not emit such light, so if we use them for illumination no fluorescence occurs and colors do not glow.
Interesting Fact
The 2008 Nobel Prize in Chemistry was awarded for the development of the GFP method used extensively in biological research. In this method, a gene producing a protein called green fluorescent protein (GFP) is introduced into living cells. It glows green fluorescent under ultraviolet light, just like in this experiment. This method allows monitoring biochemical processes occurring in the cell.
Infrared light, at the other end of the visible spectrum, is also invisible to us, but a fun experiment can demonstrate how it can be detected. See the experiment How the remote control works.