# Double Slit Experiment at Home

| April 11, 2013

It’s strange to think that one of the most famous experiments in physics — the double slit experiment — can be easily performed at home. With just a cheap laser and some imagination one can do this experiment without much hassle.

So the famous experiment that gave evidence for the wave-like behavior of light was demonstrated by Thomas Young back in 1801. The interesting fact is that, in his famous speech in front of the audience at the Royal Society of London, Young demonstrated interference of light without even using a double slit grating. Instead, Young used a thin card and the light of the sun to create the interference pattern. As he famously said: “The experiments I am about to relate … may be repeated with great ease, whenever the sun shines, and without any other apparatus than is at hand to every one.” So the method that Thomas Young is speaking about is illustrated in the picture on the left. Basically, the light passes through a small pinhole and is then split by a small card into two beams, which then interfere.

As it is discussed in this great page, the pinhole and the width of the card have to be of specific size to get reasonable interference patterns. The values given in the website are as follows: the width of the card should be about 2 mm and the size of the pinhole should be as small as a typical needle hole. So basically, all you need to do is to get a laser, fix a pinhole in front of it and use something thin to act as the card in the picture.

Obviously, a similar setup could be built by using a screen with two slits instead of a card, however, making the slits as narrow as required is not an easy task. Thus for those, who are a bit lazy of setting this up, a simpler solution would be buying a cheap laser, which come in a variety of prices and wavelengths, and a diffraction grating. For a cheap laser pointer you could try something like this. As for the diffraction gratings, they can also be found on Amazon for very affordable price.

A cheap laser and a diffraction grating to create interference

The problem with this approach is that the pattern is highly dependent on the width of the light beam and the number of slits. In the picture below you can see how the pattern depends on the number of slits. So in order to get the pattern, which is most similar to the 2 slit pattern, you need a very thin laser beam, or you could try covering most of the diffraction grating and leaving only a small slit for the light to pass through, which is not as easy as it sounds.

Alternatively, white light could be used with the diffraction grating to create a similar pattern of “rainbows”.

The great thing about the double-slit experiment is that the setup that was described above is not much different from the real experimental setup that is used in undergraduate physics labs. Below you can see a picture of such an instrument used to create interference patterns. As you can see, instead of a diffraction grating, a setup of single and double slits is used to control the patterns. The intensity of the laser is also controlled by filters and the power source. Also, the intensity of the pattern is measured by a photo diode and a photo multiplier. The full price of this instrument is about 7000 dollars.