Just about all of us were taught as kids that light is both a particle and a wave. This is an example of one of those things we "know" but really don't have any understanding of. Most of us remember this fact but either never get a proper explanation, or we breeze over it in school and forget it immediately. This really is a shame, because the details involved really are one of the most spectacular things humans have ever discovered about the universe and the bizarre reality we live in.
Wave/particle duality, quantum mechanics, Heisenberg's uncertainty principle, and superposition are all intricately entwined with each other and are really just terms used to describe separate properties of a single phenomenon. Don't let all of that technical jargon throw you, it's really not as difficult (and certainly not as boring) as it may sound. I've found a very interesting and easy to follow video explaining the incredibly interesting wave/particle duality debacle by examining a famous experiment known as the "Double Slit Experiment".
Stick around after the video for more, including an experiment you can do at home with common household items and a cheap laser pointer that allows you to play with Quantum Physics in your home!
Wow. Pretty exciting stuff huh? Well stick around, we're not through yet. The fact that simply observing the position of the electron changes it's behavior has been boiled down to what is known as the Heisenberg Uncertainty Principle. It gets a little complicated in it's details, but essentially what it means is that the more certain you can be of a particles position, the less you can be certain of it's momentum and direction of movement, and vice versa.
This seems all well and good, but where it gets strange is that when you create a situation where you can determine a particle's (such as a beam of light's) position very accurately, it's direction of movement will necessarily become proportionately more varied in order to adhere to the law. When you try to force the issue and determine both simultaneously, light (and other quantum objects) begin to behave in some very counter-intuitive ways and some strange things start to happen, as though the light were to say... "nope, not this time pal." as is demonstrated in this video.
You can replicate this experiment at home using only an index card, a sharp knife, a cheap laser pointer, and a cup.
Step 1. With a sharp knife, cut a slit in your index card from about 1 inch from the top to 1 inch from the bottom. Be sure not to cut all the way through to the top or bottom of the card. It doesn't need to be very long because we need it to remain very narrow.
Step 2. Set your cup on a table and place your laser on top so that it points sideways. Make note of where the laser dot is on the wall.
Step 3. Turn off the lights and hold the card so that the laser beam is aimed directly on the slit. Manipulate the size of the slit by bending the card and observe how the dot varies in width on the wall beyond the card, much like as in the video.
That's it! Not too difficult to pull off, and I'm sure you can find ways to improvise or improve the experiment with whatever it is you have lying around.
Why does that happen?
The reason the beam becomes wide is that by passing the light through a very narrow slit, we can precisely determine where the photons are, in terms of the horizontal plane, as they pass through the card. Our uncertainty of their horizontal position decreases as the slit gets smaller. Since Heisenberg's uncertainty principle states that the more we know about it's position, the less we can know about it's direction; the amount of variation in the direction of the photons increases correspondingly to compensate! Since we haven't confined the possibilities of where the photons may exist vertically as they pass through the slit, there is no corresponding increase in the variance in the direction of the photons vertically. They remain level with the laser and what we end up with is a long horizontal line rather than a dot or short vertical line as we would rationally assume. However, if you turn the card vertically, you'll see that the pattern on the wall turns along with it.
Want to get real crazy? If you cut another slit right next to the one you've already made... what kind of pattern do you think you'll find on the wall?