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Experiment_3

# Experiment_3 - Experiment#3 Interference and Diffraction...

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1 Experiment #3: Interference and Diffraction EYE HAZARD: never look directly into a laser! Starting with experiment #4, Please always bring a formatted high-density PC diskette with you to the lab. Purpose : 1. To study the diffraction of coherent laser light of known wavelength from a single slit. 2. To study the interference and diffraction of coherent light from a pair of closely-spaced slits. Equipment : Helium-Neon Laser Optical Bench with Single-Slit and Double-Slit Apertures Transparent Ruler Measuring Tape Part 1. Diffraction from a single slit Discussion : So far in your experiments on optics, you have been looking at phenomena (reflection, refraction, etc.) which are explained by the simple laws of geometrical optics, such as the statement that a light beam travels in a straight line in a homogeneous medium. The laws of geometrical optics are, however, only approximately true. In actual fact, light is a wave. It has a wavelength λ and a frequency f . Like sound waves, light should therefore bend around corners. This does happen, but since the wavelength of light is very small (400–700 nm for visible light) compared to the sizes of objects around us in daily life, the bending is not easily observable and the approximation of geometric optics is usually very good. But if the objects encountered by a beam of light are comparable in size to the wavelength of the light, then the effects due to the wave nature of light become observable. For example, if you illuminate a narrow slit (about 10– 100 μ m width) with coherent light and project its image onto a distant screen, you will not get the simple rectangular pattern you might expect. Instead, you will see alternating dark and bright bands called fringes, extending out to large lateral distances—the light has bent around the edges of the slit, in a phenomenon called diffraction. The fringes arise because light waves coming from different portions of the slit interfere constructively or destructively, depending on their path length differences from those portions to the screen.

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