How To Make Black Light With Iphone

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.

What is "Blacklight"?

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.

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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.

What Can You Do With Ultraviolet Light?

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: