I'm not a huge fan of Halloween, but I am a fan of finding ways to squeeze physics into an event. Here are four common Halloween effects that you might have at a party---along with a quick explanation of the physics involved. I saved the best for last.
You pretty much have to use this at your party. I think it's an official Halloween party requirement. The basic idea is to take some dry ice and plop it in water. Boom---instant cool fog effect along with bubbling water.
But what is dry ice? It's just the solid form of carbon dioxide. However, unlike water ice, dry ice doesn't melt. Instead it sublimates. This means it goes straight from a solid to a gas. If you just place a piece of dry ice on a table, after a while it will have just turned into gas and mixed with the rest of the air. No puddle.
You might think that the fog you see is carbon dioxide. Nope. This fog is actually condensing water vapor from the air. When the dry ice turns into a gas, it is cold. This cold carbon dioxide gas cools off the water vapor in the air. Cool water vapor condenses into tiny droplets of water and these droplets reflect light. This white fog looks just like a cloud because it is essentially just like a cloud.
The condensing water vapor is still cooler than the surrounding air and more dense. Not only does the water vapor in the air form this fog, but the fog sinks to the surface of the table. It makes a great effect. Oh, and you don't even need dry ice. In the image above I actually used liquid nitrogen since our dry ice machine wasn't functioning. But still, it's the same idea.
Oh, and dry ice is mostly safe. Don't touch it with your bare hands because it is super cold at -75°C (that's colder than the temperature on Hoth). The other dangerous thing about dry ice is that it expands into a gas. If you put dry ice into a bottle with a closed top, it's probably going to explode. I probably shouldn't have told you that.
Sparks are just another cool visual effect that you can use for your Halloween party---or just for fun. How does it work? The whole thing starts with a high electric potential difference (in volts). Think of an electric potential difference (which we often just call potential for short) as a big hill. If you put a ball at the top of this hill, it can gain kinetic energy as it moves down the hill. The same is true for electric charges and electric potential. As an electron moves through a potential it also increase in kinetic energy.
Now just imagine that you have a free electron that is accelerating due to this electric potential difference. It can't keep speeding up forever---something is going to get in the way. That something is probably a molecule of nitrogen or oxygen (since that's what the air is made of). When this electron collides with air, the impact can free even more electrons. More electrons means more collisions and more free electrons. Now you have an electron avalanche (that's actually what it's called).
After these electrons are freed, you have both electrons and positive ions (of oxygen and nitrogen). When an electron meets back up with an ion, you get light. That's what we call a spark. You might think that this is pretty cool stuff---but wait! There's something even cooler. Where does that first free electron come from that starts this whole avalanche? The answer is space. The free electron is most likely produced by cosmic radiation that ionizes air. Without this free electron, there would be no spark (and maybe no Halloween).
Two notes: First, this idea of cosmic rays being responsible for sparks comes from the awesome introductory physics textbook Matter and Interactions. Great book. Second, if you create a high voltage source to make great sparks don't blame me if you shock yourself. Getting shocked is no fun.
How do you make a ghost? This is an old trick, but it's still great. The basic principle is that glass can both reflect and transmit light. Of course you already knew this. When you try to look outside of your house through a window at night, all you really see is your own reflection. This is because the light coming in from outside is so much dimmer than the reflected light---but light is still coming in.
So here's what you do. Take a piece of glass and place it at a 45 degree angle with respect to a viewer. Put the normal objects straight in front of the viewers so that they can be seen through the glass. Now place a "ghost" (but not a real ghost) in a location such that a reflection from this ghost makes it appear near a normal human. Clearly I am going need to include a diagram.
Hopefully it's clear that those white arrows show the path of light. Also, I replaced the audience with a camera---it's essentially the same thing. The camera (or our eyes) can't really tell where the light came from. We just trace it back to its apparent source which would be behind the glass. It's the same thing as a normal mirror except that you can also see the human through it.
Warning: awesome physics ahead. Now that you have been warned, let's get started.
When an electron in an atom is excited, it jumps up to a higher energy level. When this electron then goes back to the ground state it creates light. The frequency of this light is dependent on the size of the energy level jump. OK, but there are some special cases. The first is called fluorescence---yes, just like a fluorescent lightbulb. In this case, an excited electron doesn't go straight back to the ground state energy level: It takes more than one transition to fall down. In the fluorescent lightbulb, this happens in the powdered coating in the tube. The gas in the tube produces ultraviolet light that excites electrons in the powder. But the electrons take multiple transitions to get to the ground state and in the process produce light with lower frequency (visible light).
Stuff that glows in the dark is called phosphorescent. This is just like the fluorescent material except for one difference---one of the transitions to get back to ground state is a "forbidden" transition. This doesn't mean it can't happen, but just that it doesn't happen right away.
So here's what happens. You excite the electrons in the phosphorescent material and they jump to some higher energy level. Next, the electrons take more than one transition to get back to ground state---but one of these transitions takes a while. This means that after the light is removed from the material, it still continues to give off its own light for some time. It "glows in the dark."
The light that excites these electrons must be higher frequency than the light it gives off. In the above image, you can see a violet laser pointer will get the glow in the dark material excited. But what about a green or red laser? Nope, those won't work. The energy transition depends on the frequency of light and those lasers don't have a what it takes to get the stuff excited.
But really, you should try that blue laser with the glow-in-the-dark stuff. It's awesome.
