Young’s experiment

We shall begin with the principle of superposition, illustrated by Young s experiment (Fig. 1). If two pinholes separated by a in an opaque screen are illuminated by a small source S of wavelength A, under certain conditions it is observed that the intensity in front of the screen varies cosinusoidaUy with the angle 6 according to... 

A useful concept in understanding interference is that of coherence. Consider Young s experiment again (this time with an arbitrary source S which need not be monochromatic illuminating the pinholes). The optical disturbance at point r and time t ean be written alternatively as a sum of the disturbances at the pinholes, with propagation faetors. 

Thus far we have been considering—in Young s experiment, Michelson interference, and even measurements of mutual intensity—what is essentially two source interference. We now turn to multiple source interference. For example, a diffraction grating divides up a beam into an array of sources, which then interfere. By the principle of superposition, the disturbance in the far field is the sum over the sources... 

The wave interpretation of the interference pattern observed in Young s experiment is inconsistent with the particle or photon concept of light as required by Einstein s explanation of the photoelectric effect. If the monochromatic beam of light consists of a stream of individual photons, then each photon presumably must pass through either slit A or slit B. To test this assertion, detectors are placed directly behind slits A and B and both slits are opened. The light beam used is of such low intensity that only one photon at a time is emitted by S. In this situation each photon is recorded by either one detector or the other, never by both at once. Half of the photons are observed to pass through slit A, half through slit B in random order. This result is consistent with particle behavior. 

Young s experiment is shown in Fig. 10. AB is a thin metal plate with a short, very narrow slit S in it. CD is another... 

Suppose, however, that the plate is only exposed to very weak light for a very short time such as one millionth of a second. In this case we can imagine that very few photons, only one for example, arrive at the two equal areas at A and B. If only one photon arrives, it must fall either on A or B, so that it is not possible for the number falling at A to be ten times that at B. Thus we see that the number of photons is only proportional to the intensity when large numbers are considered. If we consider a single photon in Young s experiment, then it may fall anywhere on the screen, but we suppose that the chance of its falling on any small area is proportional to the intensity of the waves at that area. 

Thomas Young s in 1803 performed for the first time the classic experiment that demonstrates optical interference the two-slit interference experiment, which appears as an example in many books on optics in order to explain the concept of interference. Young was not the first to report the phenomenon. It had been observed in various forms, such as Newton s rings, Brewster interference, and Michelson interferometer. Young s experiments however mark a point in the history of science. They led the way to the studies of Augustin Fresnel (1816), who introduced the measurement of the wavelength of light and established... 

Referring to Young s experiment (Fig. 2.24) with a narrow bandwidth but extended source, spatial coherence effects will predominate. The fringe pattern in the plane B will depend on IXSx, S2, t) = Pnit). In the region about the central fringe (r2 — ri) = 0, t = 0, the values of ri2(0) and yi2(0) can be determined from the visibility of the interference pattern. 

Light, including infrared radiation, is an electromagnetic wave. Due to the wave nature of light, interference occurs between two light waves of an identical frequency when they overlap each other. Young s experiment, which is known as the first demonstration of the... 

Young s experiment, (a) Optical arrangement and (b) interference fringes on... 

Figure 4.2 Interference of light waves in Young s experiment, (a) Propagation of iight waves from two slits, 5, and S2 and (b) light intensities on Screen.

The qualitative explanation of Young s experiment given above may be formulated in the following way. The distances from Sj and S2 to a point (designated as P) located between the Slits and the Screen in Figure 4.1a are denoted, respectively, as / i and / 2- If P exists on the thicker lines in Figure 4.2a, the following relation holds ... 

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