Ultraviolet (UV) light is invisible to the human eye, but gets a lot of attention. After all, UV light can cause sunburns, suntans, or skin cancer. It can fog photographs, make objects fluoresce, or even attract butterflies to flowers (they can see into the UV spectrum). Focused into a laser beam, it can be used for precision cutting in machine or surgical applications.
Some materials allow UV light to pass through (UV-transparent) while others block UV light (UV-blocking) either fully or partially. It’s pretty obvious that a slab of concrete blocks UV, but what about glass? Glass looks clear, sure enough, but if you can’t see UV light in the first place, how can you tell whether it is getting through?
Once you’ve gotten all your equipment together, the experiment is simple. You’ll shine UV light on each material in turn, and see if you can detect UV light on the other side.
#1. UV Light Source
There are several options, you’ll make your choice based on what you have available and what your budget is.
If you have access to a UV-Vis spectrophotometer with an open pathway (like the 8453 – a common model in labs), you’re in luck. This not only gives you the ability to scan individual ultraviolet wavelengths, but also comes with the detector built in.
A second option is a UV laser. This gives you the advantage of an intense beam that you can easily aim, but if you don’t already have access to one, it’s probably not worth the investment.
A UV lamp is the next option – these are available in long and short wavelength UV. This is nice if you want to test materials at two different wavelength ranges. A couple different UV lamps can be found at Scientific, among other places. If your lab does any thin layer chromatography, it probably has one.
The final option is a simple black light. You can find this at Scientific, science stores, and sometimes even large shopping centers. For that matter, ask your parents. They may have one stashed away from the 1980’s.
For any light source, if the light is too intense and registers off scale for your detector, you can put a neutral density filter in front of the light source to tone it down.
#2. UV Protection
UV light WILL damage your eyes. Wear UV glasses at all times your light source is on. Scientific has cheap UV goggles available, but there are many different models of both glasses and goggles out there. Be safe, you won’t be able to see your results if you burn your retinas.
Similarly, be smart and wear long clothing to block the UV light. Slap some high SPF sunscreen on exposed skin. The raw UV light you’re working with is more intense than what gets to Earth from the sun, and will burn your skin faster.
#3. UV Detector
As with the light source, you have a number of options.
If you are using a spectrophotometer, a detector is also built in.
You can build a detector similar to the spectrophotometer’s by using a photo-multiplier tube or a diode array detector (among others). These are pricey, and require a knowledge of electronics to connect, so unless you have both the equipment and the expertise, skip this option.
Photographic film makes a good UV detector. UV light will expose the film. It will fog black and white film, and turn color film blue. Film is only one use, so you’ll need a piece for each individual experiment. Fortunately, you have the option of cutting film into smaller pieces. Remember – you’ll have to work in a dark room to avoid exposing your film, and keep the film protected from light until you’re done with it. There are cameras designed for working with UV light, but not all are. If the camera has UV filters, you’ll need to remove them to see any results. A simpler option might be to build your own pinhole camera for this experiment. (There are also digital cameras that can detect UV.) Remember that exposure time must be consistent for each experiment if you want to be able to compare results.
There are UV indicator cards available. Designed to monitor how much sunscreen you need, they show whether UV levels are low, medium, or high. They are reusable, and can be used as a detector in this experiment. Among other places, Scientific has these too.
UV light will make fluorescent objects glow. Fluorescent ink (neon markers / highlighters) on a piece of poster-board can be used as a detector. You won’t get intensity information, but it will at least provide a yes/no result.
You prevent stray UV light from getting to the detector, you need a large, UV-blocking wall that has only a small hole in it where different materials can be tested. A thick piece of cardboard will do the trick.
#5. “Dark Light”
To be able to see what you’re doing, you’ll need some light. The light from a dark room is ideal. If you can’t manage that, you can use dim incandescent lighting. (Fluorescent lights give off UV light – avoid them). You can use a curtain to help block light from the light source. In particular, try to prevent light from falling directly on your detector.
#6. Sample Materials
You’ll have to decide what you want to test, but here are some ideas:
Borosilicate Glass (like Pyrex, the stuff beakers are made of)
Cellophane / plastic wrap
Cotton fabric (varying thicknesses)
Other fabrics (Nylon, Rayon, Wool, Linen…)
Calcium chloride crystal
Sodium chloride crystal
High band-pass filter
Low band-pass filter
Ice (a clear, thin sheet)
Assemble your equipment in the order Light source, Sample material, Wall (the opening should be aligned with the light, sample and detector), Detector. Make sure that distances are the same throughout the experiment. The intensity of light decreases with distance, so to be able to compare results, you have to keep distance constant.
For each material in turn, measure whether (and how much) UV light reaches the detector. For most detectors, exposure time is not critical. If you are using film, however, be sure to make the exposure short and of constant duration. Cameras with shutters already do this. This is important, since length of exposure is just as important as light intensity for film.
Record all your data for each material. Remember to list not only the material and detected light intensity, but also parameters such as exposure time, distance, thickness of material, etc.
Evaluate your data. Which materials blocked UV? Were some better than others? You may want to rank them from best to worst at blocking UV.
Consider where these materials are commonly used in real life. Are their UV properties one reason for being used in that situation? Are there places you can think of where using a different material to block UV might be better?