What is a blacklight and how do you make one? This is the topic of a recent MacGyver episode in which he quickly creates an improvised blacklight to find hidden messages on a wall. You can watch the scene here---and a disclaimer, I am currently the Technical Consultant for the show. But still, there's lots a great science in this one little scene.
OK, it's not really a black light. It's better to call it what it is: ultraviolet light. Let's start with a quick overview of light. Of course, light is an electromagnetic wave (oscillating electric and magnetic fields), but in this case the frequency is the important aspect. For some narrow range of frequencies, the human eye can detect these waves---this is called the visible spectrum. The lower frequency waves are interpreted by our eyes as the color red, and the higher frequency would be violet.
Here is a picture that might be useful.
Of course, you could break this spectrum of colors into seven parts: red, orange, yellow, green, blue, indigo, and violet. But what the heck is indigo? Really, you could break this into just three colors---red, green, blue---or a thousand colors if you prefer. I tell my students there are seven colors because that's how many Isaac Newton wanted there to be. Seven is a cool number, and back in Newton's day there were just seven regular moving objects in the sky: the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. Fun fact: That's the same order as the days of the week that are named after these objects. Save that for a party (along with radioactive bananas).
If you combine all of these colors of light together, your brain detects that as white light. If there is no light going into your eye, your brain interprets that as the color black (that's why a completely dark room looks black). But what about the infrared and ultraviolet on the sides of the spectrum? Their names and placement in the spectrum can be explained by their discovery. In 1880, William Herschel took white light and split it into the rainbow colors with a prism. He found that if he put a thermometer in the section past the red color of light, it would still warm up. There must be some type of light that humans can't see, but that still heat up the thermometer. Since it was below red, he called it infrared. The same is true for ultraviolet.
Surely you've seen a UV light. They used to be popular at parties because they would make some materials on your clothes look like they were glowing. Also, the UV lights are used to detect different materials---like at a crime scene or in an escape room. But how does this work?
The key to a useful UV light is fluorescence. But first, let me just talk electrons in matter. It turns out that electrons in a bound system can only be at certain energy levels. When an electron goes from a higher to lower energy level light is produced. Further, the frequency of this light is proportional to the change in energy levels. This can be written as:
The h is known as Plank's constant---but that's not really important right now. Typically an electron will make the quantum leap (see what I did there) from one excited state to the ground state---just one jump making one color of light. However, for some materials, electrons make multiple transitions to ground state. For each transition down, they produce light---different frequency light. So here's what happens. Some light is incident on the material and this excites an electron. The electron then takes multiple down transitions which produces different colors of light than the one that excited it. This process is called fluorescence.
Of course there's a catch. In order to get fluorescence to work, you have to start with a higher frequency light---like violet or ultraviolet. But if you shine this on some materials, it will produce light of lower frequency. UV light in, visible light out.
Here is an example of fluorescing highlighter marker on a wall. If you look with just visible light, you don't see much. With the lights out and UV light shining on it, the highlighter fluoresces and you can easily see it.
This is actually how a fluorescent light works. For the traditional fluorescent tube lights (and the compact fluorescent lights), an internal gas is excited by accelerating electrons. This excited gas then produces UV light. On the inside of the tube is a white powder coating that is fluorescent. The UV light hits the coating when then fluorescent and produces white light (many different colors mixed together).
Now we finally get to the MacGyver hack. Could you make a UV light with the LED flash on a smart phone? The answer is ... maybe. In order to understand this hack, you need to understand how an LED works. The Light Emitting Diode is indeed a diode---a solid state device. The LED produces light in a similar manner to excited electrons in a neon gas tube (you've seen these neon signs). However, for the neon light the excited electrons change energy levels at the atomic level. In an LED, the electrons change energy levels in a solid state material. Really, that's the only difference. But this means that the frequency of light produced from an LED depends on the value of this energy transition. You only get one transition and therefore only one color of light.
Then how do you make a white LED light? They are everywhere, now---but how do they work? Of course you could get a red, green and blue LED and combine them together to make a white light, but that's not how most of them work. Instead, a white LED is either a violet or ultraviolet LED with a fluorescence material. The LED produces a high frequency light (either violet or UV) and this makes the material fluoresce to produce other colors (lower frequency).
Since this fluorescent material isn't 100 percent efficient, some of the UV light could pass through and be mixed in with the white light. If you want to take a white LED and get UV light, you just need to block the visible colors while leaving the UV light to pass through. There are some materials---like fused quartz or fluorite that do exactly this and can be used to make some pretty cool UV photographs. But could there be other materials that might do the job? Maybe. In the MacGyver episode, he uses the floppy disk part from inside a 3.5 inch disk (kids these days only know this as a "save icon"). Different floppy disks uses different materials and this round disk could possibly allow UV to pass while blocking visible light.
How about a quick review. Here's how you make a UV light with a smart phone.
- Start with a smart phone that has an LED light (for the camera flash). You want two things from this light. First, it should be a UV LED with fluorescent material and second it shouldn't be 100 percent efficient.
- Next find some material that blocks visible light but not UV---this might require some trial and error.
- Turn off the lights. Why? If you don't the UV light will still fluoresce some stuff that you want to look at, but you won't be able to tell since there will be all this other visible light reflecting off stuff.
Clearly this won't work with any old light or material, but it's at least plausible.
I am going to show you a pretty cool demonstration of fluorescence. All you need are some laser pointers---red, green, and blue (they are at least much cheaper than they used to be). Let me start with the red laser pointer. I'm going to take it and shine the red light on different things I can find. You should see that no matter what you shine this red laser on, you get a red dot.
There should be no surprises with the red laser. You don't see any fluorescence because the frequency of red light is too low to make these higher energy transitions. But what about a green laser? Take your green laser and shine it around the room. You are looking for things that have a non-green dot on them. In particular, try shining the green laser on orange plastic stuff. This is what you can see.
Notice that for some materials the laser dot isn't green? Yup, that's fluorescence. Now for the blue laser with an even higher frequency.
Notice that again the blue laser causes fluorescence---but with a higher frequency light, it can cause the effect in a wider range of materials. Go ahead and look for other materials that cause fluorescence. Use the blue laser since you will be able to find more stuff. You might be surprised to find things like olive oil and some wines will do this.
