O1_Manual - General Physics II Lab O1 Two-slit Interference...

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General Physics II Lab O1 Two-slit Interference & diffraction grating General Physics II Lab O1 Two-slit Interference and Diffraction Grating Purpose In this experiment, you will study the interference pattern generated by two closely spaced slits and use the two slits and multiple slits (a diffraction grating) to determine the wavelength of visible lights. Equipment and components Optics bench, light source, diffraction plate, diffraction grating, diffraction scale, three color filters (red, green and blue), ray table base and slit mask. Background Light is a transverse electromagnetic wave with a wavelength, , which is directly linked to the color of visible light. An important experiment to demonstrate the wave nature of light is the two-slit interference experiment.The essential geometry of the two-slit experiment is shown in Figure 1. A coherent light passes through the central slot of the diffraction scale and falls normally on the diffraction plate, which is an opaque screen with two closely spaced, narrow slits, A and B. According to Huygen’s principle, each slit acts as a new source of light and the wavelets from each slit will interfere and give rise to an outgoing light, whose intensity varies with the angle θ relative to the incident direction. At the central symmetric point ( θ = 0), where the zeroth maxima is located, light rays from slits A and B travel the same distance from the slits to your eye, so that they are in phase and interfere constructively on your retina. At the first order maxima (to the left of the viewer) light from slit B travels one wavelength farther than light from slit A, so that the rays are again in phase and constructive interference occurs at this position as well. At the nth order maxima, the light from slit B travels n wavelengths farther than the light from slit A, so again, constructive interference occurs. λ Figure 1 Geometry of the two-slit interference experiment In Fig. 1 the line AC is constructed perpendicular to the line PB. Since the slits are very close together (not shown to the scale in Fig. 1), lines AP and BP are nearly parallel. Therefore, to a very close approximation, AP = CP. This means that, for constructive interference to occur at P, it must be true that BC = n λ . From the right triangle ACB, it can be shown that BC = d sin θ ’, where d = AB is the distance between the two slits on the diffraction plate. Furthermore, one can readily show that
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This note was uploaded on 03/31/2011 for the course PHYS 1 taught by Professor Nianlin during the Spring '11 term at HKUST.

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O1_Manual - General Physics II Lab O1 Two-slit Interference...

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