It can be tempting to try to compartmentalize the teaching of light to simplify down the abstract components into simpler and more approachable constructs. But the art of light should always shine when teaching about the topic. While we are limited to seeing only visible light, we can see evidence of the other kinds. Microwaves can be used to expand a bar of Ivory soap or melt a chocolate candy bar in systematic fashion. Older cell phone cameras can detect infrared light from remote controls and ultraviolet light causes many chemicals to fluoresce and produce visible light. The brown spots of a banana, the chlorophyll pigment extracted from spinach leaves, quinine (tonic water), used or new glow sticks, laundry detergents and many more things will fluoresce visibly under ultraviolet light.
One of the best demonstrations of ultraviolet light is paper currency. While the exact chemicals used are not disclosed to prevent counterfeiting, one can find one of the general blueprints1 if you find the right source. Last year I had students and teachers bring in as many different foreign paper currencies as we could find. I then recorded the front and back of each currency under a black light. The intricate patterns and imagery used varies between countries. The best ones that made the final video were India, Singapore, China, Sweden, Australia, Japan, Cambodia, Myanmar, the Euro, Budapest, Venezuela, Saudi Arabia, Canada, Kenya and the United States of America. The video can be found here. Two of my favorites were Sweden and Saudi Arabia. Sweden has something written that I have not been able to find someone to interpret yet. Saudi Arabia has a picture of former King Fahd2 not only on the bill itself, but also an image of him is done in the fluorescent dye that emerges under the glow of the ultraviolet light.
While there are many obscure (scorpions and vaseline glass3) and accessible (driver's license) items that fluoresce under ultraviolet radiation, having students bring in foreign currency is a fun way to engage students in the demonstration. Much of the money brought in was lower in value and so there were no huge concerns from students or parents about mixing up money and those that did came prepared with labeled envelopes. There usually is variation depending on value although not always. This demonstration can be used for a variety of topics, but it is important regardless of level that students “see” the other forms of light when possible. All light has the same composition but varies in frequency, wavelength and energy. In fluorescence the light incident on the material is higher in energy and frequency than the light emitted. The electronic excitations are followed by the electrons shifting to a lower excited state before falling back to the ground state. Thus we are seeing ultraviolet light incident on the paper money and blue or green or red light emitted back. Perhaps you care to expand on currency to the magnetic ink used or the cost of making a penny. Whether you aim to tie this into photoelectric effect, calculations relating frequency and wavelength, demonstrating the seven classifications of light or something completely different is up to you. Feel free to share any cool findings.
- https://en.wikipedia.org/wiki/Saudi_riyal accessed on 7-29-2015
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