Thomas Young’s Double Slit Experiment

   The Double Slit experiment was first performed by the English scientist Thomas Young’s in the year 1801 in an attempt to resolve the question of whether light was composed of particles, or consisted of waves traveling through ether. The experiment consisted of a light source that illuminates a thin plate with two parallel slits cut in it, and the light passing through the slits strikes a screen behind them. The wave nature of light causes the light waves passing through both slits to interfere, creating an interference pattern of bright and dark bands on the screen. The interference patterns observed in the experiment seemed to discredit the particle theory, and the wave theory of light remained well accepted until the early 20th century. When evidence began to accumulate which seemed instead to confirm the particle theory of light. The experiment was not performed with anything other than light until 1961, when Clauss Jönsson of the University of Tübingen performed it with electrons.

    In the quantum variation of the double-slit experiment detectors are placed in either or both of the two slits in an attempt to determine which slit the photon passes through on its way to the screen. Placing a detector even in just one of the slits will result in the disappearance of the interference pattern. One photon at a time can pass through the apparatus with the identical result of interference fringes. It should be noted, however, that strictly speaking, no experiment with a true single-photon source was done until 1986. If either slit is covered, the individual photons hitting the screen, over time, create an ordinary diffraction pattern. But if both slits are left open, the pattern of photons hitting the screen, over time, again become the series of light and dark fringes. Something strange happens when you try to determine through which slit each particle passed: the interference pattern disappears. Imagine using excited atoms as interfering objects. Directly in front of each slit, having a special box that permits the atoms to travel through them. Each atom therefore has a choice of entering one of the boxes before passing through a slit. It would enter a box, drop to a lower energy state and in so doing leave behind a photon (the particle version of light). The box that contains a photon indicates the slit through which the atom passed. Obtaining this "which-way" information, however, eliminates any possibility of forming an interference pattern on the screen. The screen instead displays a random series of dots, as if sprayed by shotgun pellets. But what if you could "erase" that telltale photon, say, by absorbing it? Would the interference pattern come back? Yes, predicted Marlan O. Sculley of the University of Texas and his co-workers in the 1980s, as long as one examines only those atoms whose photons disappeared. In a complicated setup, Researchers are using photons rather than atoms as interfering objects.

If You are looking for a simulation you can download it here. It has a built in double slit experiment or you can import the code below this section.

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Ideally we could perform the experiment using an optical bench like the one shown above. These benches are not a dime a dozen. (If you have an optical bench you probably don't need my help setting it up, but if you do I can direct you to a site that will help. ) Because of the high price a less costly way of performing the experiment is shown below.

To perform the experiment (almost) as Thomas Young did, is a relatively simple matter. Every thing you need for the equipment is listed below.

A laser. I recommend a green laser, Because of the high frequency output, But a simple red laser will do.

A Small piece of card stock.

A sheet of white construction paper.

A hobby knife

A small book

Warning Do not look directly into the laser. It causes permanent damage.

To setup the experiment cut two slits into the center of the card stock with the hobby knife. Make sure they are no more than 1 mm apart and about 2 mm in with, and they can range form pinholes to 7 mm in length. Then cut a slit about ½ inch in length at the bottom of the card just below the slits you made. Bend the half of the bottom of the card stock over one way and half the other way. Tape it to the table. Cut an 1 inch slit into the construction paper half way down the long side. Then fold the bottom of the paper the same way you did the card stock. Tape Paper to the table directly inline with the card stock about 6 inches away. Place your laser on the small book on the other side of the card stock Line it up with the slits then adjust your distance slowly until you see the interference pattern.

Tips

 

Make the room as dark as possible. This includes the computer screen.

The importance of a precision cut double slit can not be stressed enough . If you do not produce an interference pattern then try cutting smaller slits. The size of the slits will very depending on the type of laser. I have read that 1 Micron with 25 Microns of spacing is ideal.

For more advanced users. I have come up with a quantum version of the experiment. This requires parts that are harder to come by. Because of this I have not preformed the experiment. The difference between this experiment and the one above. I recommend that you acquire more durable pieces. Take the laser and pass it through a beam splitter. There are several types, the crystal type looks the best, But in order to use this you will need an ultraviolet laser. Redirect the beams with a set of small mirrors. To a second crystal. That recombines the two beams into one then pass it through the slits onto the screen. This should produce the interference pattern. By designing the test this way you I think that if you were to place a polarized filter on one of the beams after the first crystal then it should tag the photon and collapse the wave function.