Similarly, if the paths taken by the two waves differ by any integral number of wavelengths ( λ, 2 λ, 3 λ, etc.), then constructive interference occurs.įigure 5. More generally, if the paths taken by the two waves differ by any half-integral number of wavelengths, then destructive interference occurs. If the paths differ by a whole wavelength, then the waves arrive in phase (crest to crest) at the screen, interfering constructively as shown in Figure 4b. Waves start out from the slits in phase (crest to crest), but they may end up out of phase (crest to trough) at the screen if the paths differ in length by half a wavelength, interfering destructively as shown in Figure 4a. Thus different numbers of wavelengths fit into each path. Each slit is a different distance from a given point on the screen. To understand the double slit interference pattern, we consider how two waves travel from the slits to the screen, as illustrated in Figure 4. (c) When light that has passed through double slits falls on a screen, we see a pattern such as this. Wave action is greatest in regions of constructive interference and least in regions of destructive interference. (b) Double slit interference pattern for water waves are nearly identical to that for light. We can only see this if the light falls onto a screen and is scattered into our eyes. These waves overlap and interfere constructively (bright lines) and destructively (dark regions). (a) Light spreads out (diffracts) from each slit, because the slits are narrow. Double slits produce two coherent sources of waves that interfere. Figure 2 shows the pure constructive and destructive interference of two waves having the same wavelength and amplitude.įigure 3.
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We illustrate the double slit experiment with monochromatic (single λ) light to clarify the effect. Young used sunlight, where each wavelength forms its own pattern, making the effect more difficult to see. Why did Young then pass the light through a double slit? The answer to this question is that two slits provide two coherent light sources that then interfere constructively or destructively. Incoherent means the waves have random phase relationships. By coherent, we mean waves are in phase or have a definite phase relationship.
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Furthermore, Young first passed light from a single source (the Sun) through a single slit to make the light somewhat coherent. Why do we not ordinarily observe wave behavior for light, such as observed in Young’s double slit experiment? First, light must interact with something small, such as the closely spaced slits used by Young, to show pronounced wave effects. Without diffraction and interference, the light would simply make two lines on the screen. Here pure-wavelength light sent through a pair of vertical slits is diffracted into a pattern on the screen of numerous vertical lines spread out horizontally. Photons having more energy than this cannot be produced as the probability of two electrons giving up their kinetic energy to produce one photon is very. However if all all the kinetic energy of one electron was converted into one single X-ray photon this would represent the maximum energy and hence maximum frequency $f_$ In general not all of the electron's kinetic energy $eV$ is converted into a single photon.
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When the high energy electrons hit the metal target on the anode they are slowed down very rapidly and in doing so emit electromagnetic radiation (photons). The kinetic energy of these electrons is $eV$ where $e$ is the charge on the electron. The electrons are first accelerated by being attracted to a positive anode which is at a high potential $V$ relative to the negative cathode from which they are emitted. The X-rays are produced by getting energetic electrons hit a metal target.
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Your graph is a standard one to show the spectrum of wavelengths emitted from an X-ray tube.