Diffraction and Interference

Is light a wave or a particle? The Dutch physicist Christiann Huygens (1629-1695) advocated that light was a wave. Huygens principle is often used to describe how waves in general propagate. In 1704 Newton published the first edition of his book Optiks in which he championed the notion that light consisted of particles which we now call photons. Thomas Young performed a two slit interference experiment in 1801. The dark bands observed by Young can only be explained by destructive interference, a wave, not a particle, property. Maxwell’s equations, with their wave solutions, also support a wave theory for light. On the other hand in the early 20th century there were phenomena involving light that could only be explained by a particle theory for light. Examples are the photoelectric and Compton effects. Is light a wave or a particle? It is both, but not at the same time. Whether the particle or wave description is used depends on what question is being asked. Questions about the propagation of light require a wave theory. Some, but not all, questions about the interaction of light with matter require a particle theory. This conundrum is commonly called wave-particle duality. In this experiment you will examine what happens when a plane coherent monochromatic beam of light from a laser is incident on single slits of various widths, and double slits of various widths and separations. The former is called single slit diffraction, and the latter is called two slit interference. The two terms are very much the same physical phenomena, with diffraction being very much part of any multi-slit interference pattern, and interference between waves from various parts of a single slit producing the single slit diffraction pattern. These patterns, which are produce by propagating light, can only be explained by a wave picture of light. This experiment is greatly facilitated by the use of a laser as a light source. The laser light is highly monochromatic and coherent. Coherence means that there are surfaces, in this experiment approximately planes, in which the light is in phase. When such light falls on a slit or slits, the electric field ~ E oscillates in phase over the area of the slit(s) and Huygen’s principle can be used to calculate where the transmitted light will interfere constructively and destructively. Young did not have a coherent light source. To achieve enough coherence to see fringes, he had to use a single source slit for his two slit experiments. This single slit was narrow enough to produce the coherence needed. It also gave him very much less intensity than you have at your disposal with the laser.