In the early 1800s, Thomas Young darkened a room and projected a light through a screen with two slits cut parallel to each other. He expected to see two bright spots on the opposite wall from two streams of light traveling through both slits. Instead, he saw a series of bright spots separated by dark spots. The results of this experiment forced scientists to think of light as a wave.
The idea of light as a wave seems common sense now, but at the time of Young's revelatory experiment, physicists were mystified by the concept because they could not determine what light oscillated. The results of the double-slit experiment gave credence to light's waves properties because the dark and light spots were produced as a result of wave interference, in the same way that dropping two stones near each other in the same pool of water will cause constructive and destructive overlapping ripples. Scientist James Maxwell finally determined through a series of experiments in 1873 that light oscillates electric and magnetic fields.
Proposed in 1973 by James Wheeler, the delayed choice experiment was intended to prove that the method of detection could change after a light particle passed through the slit, delaying its choice and revealing whether light will behave as a wave or a particle in a particular situation. The technology to perform the experiment remained unavailable until 2007, when physicists were finally able to confirm that Wheeler's hypothesis was correct. This information is particularly useful to scientists working in the field of quantum physics.
Science students can conduct their own double-slit experiment with materials that can be found at home or any general purpose store. Painting a cardboard box black will substitute for Young's darkened laboratory. Students need to cut the edges on one side panel so it opens as a single flap and completely remove the opposite box side. The removed box section will serve as the double-slit screen, so two 1-inch parallel slits should be cut approximately 1/2 inch from each other before the panel is reattached to the box in its original position. A large piece of black fabric will cover the box and the hand of the person conducting the experiment. All light in the room should be extinguished with the exception of a flashlight aimed through the slits toward a blank wall.