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Christmas Laser Lights

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No ladders, no time-consuming decorating - a laser light projector is a godsend if you want hassle-free decor. All you need to do is stake it into the ground, turn it on at the wall and you 've got sparkles in seconds.

Christmas lights on the outside of your house might look awesome, but they really aren't that much fun to hang up. This year, we are using something different-lasers. Yes, you can now add Christmas lights to the outside of your house with lasers. The basic idea is to stick a small device in your yard that shoots lasers at your house to make some spots. These spots then look like little bitty lights. When Christmas time is over, just pick up the laser out of the yard. It's not quite as bright as your traditional lights, but it's so simple to set up.

The first element of this device is the laser. I'm not going to talk about lasers today (but I will in the future) except to say the following:

They mostly just produce one color of light (and thus one wavelength). So, a red laser creates light with a wavelength of something like 650 nm. We call this monochromatic. The light produced is coherent. This means that all the light produced is in phase such that you could think of it as just one wave (with one wavelength).

The laser light is collimated-such that it mostly just goes in one direction with very little beam spreading. Yes, these are just approximately true-but let's just work with this for now.

So, how do you take one laser and with it make many spots on your house? The answer is a diffraction grating-which is essentially a device with many super tiny lines on it. It turns out that when a wave (like light) passes through an opening, it sort of re-radiates. We call this diffraction.

But if waves bend through openings, wouldn't that mean you could see around a corner of a doorway? In theory, yes. In practice-no. It turns out that the amount of bending for waves that travel through an opening depends on both the size of the opening and the wavelength of the wave.

You only get noticeable effects when the size of the opening is around the same size as wavelength. Since red light has a wavelength of 650 nanometers, you need really small openings.

But what happens when two waves from different sources are at the same place and the same time? This is what we call interference. There are two extremes of what can happen. First, there is constructive interference. If both the waves are in phase such that their peaks are lined up, then the amplitudes of these waves will add together and produce a wave with twice the amplitude.

Diffraction Grating
There are all sorts of ways to get light to create an interference pattern when it shines through an opening. Let me just consider the method of using multiple openings. This is usually accomplished by using a series of super tiny lines on a glass plate-called a diffraction grating.

If we have light going through a bunch of openings, then this light will diffract in such a way that it will expand as it goes out of the slit. This light can be represented as different light rays. By changing the angle that light comes out of the slits, you would get different path length differences. Light will constructively interfere when this path length is an integer multiple of the wavelength of light. With a little bit of trig, we get the following expression.

Alternatives for Your House: Outdoor Lasers and LED Projections
What is Christmas without beautiful holiday lights brightening up the neighborhood? I thought that the old style, large-bulb outdoor Christmas lights don 't exist anymore - but, wait, they do?! Not only are they surprisingly still available in stores, the house I live in came with these lights pre-installed and we have yet to take them down.

How convenient to have these lights on the house ready to go-we can just plug them in, right? Unfortunately, each of those bulbs uses around 6 watts every hour, which means my 100-light string may be using 600 watts per hour! Our laziness is not only costing us money, it has big energy costs as well.

Besides LED Christmas lights and automatic timers, there are new alternatives to string lights that are even better for outdoor use. The first option is lasers. Not only are these very easy to install, because you simply stab them into the ground and point them at your house, but they only use approximately 0.005 watts per hour.

You can also purchase laser lights with programmed patterns and a variety of twinkly designs, eliminating the need for other decorations. The one thing lasers can 't do, however, is make white light. Lasers always come in red, green, blue, and other bright colors, so keep this in mind when planning your holiday display.

If you really want a classic look, there are now LED projectors that will project a large image onto a wall, such as snowflakes, reindeer, and stars that move in a rotation. These devices do make white light and generally have four bulbs using around 6 watts per hour. However, since you are only using one of these rather than a whole string, they still cut back substantially on energy usage.

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