4.8.1: Introduction to Young's experiment

Young's experiment demonstrates interference of waves from two similar sources. It is a classic demonstration of the interference and of the nature of waves. Here we look first at Young's experiment using water waves, where the displacements due to the waves can be seen directly. Then we compare with Young's experiment using laser light. This page supports the multimedia tutorial The Nature of Light. Young's experiment with water waves Young's experiment with laser light Young's experiment: the geometry Comparison: Young's experiment with water waves and with light

Young's experiment with water waves Young, a contemporary of Newton, performed his celebrated experiment with light, to demonstrate its wave nature. Here, we'll look first at a similar experiment using water waves, for which the displacements are visible. Two pencils attached to a frame are being sinusoidally vibrated in the vertical direction. They touch the water and create waves that spread out radially.     Young's experiment uses two similar sources to demonstrate interference: here in water waves. In the upper view, on the axis of symmetry, we can see constructive interference: along this line, the combined waves from the two sources has maximum amplitude. This is marked by a red line on both the upper and lower views. This is called constructive interference and it creates an antinode in the wave pattern. A little to the left of that line, we can see a line where the wave combination hardly disturbs the water at all: destructive interference or a node. This is marked by a blue line on both views. Along this blue line, the distance from the two sources differs by half a wavelength, hence the destructive interference: the waves arrive there half a cycle out of phase. To the left of both lines, there is another line of antinode, again marked with a red line. Along this line, the distance from the two sources differs by one wavelength. The pattern of nodal and antinodal lines continues all the way around the two sources. The two different views are of the same apparatus, taken from different angles. In the upper shot, we see the waves on the water surface. On the lower, we see the distribution of light intensity due to the refaction of light by the waves. In the experiment above, the clip is cycling over seven frames. For this frequency, the lower view is not very clear. For that reason, we show below a slightly higher frequency.   Young's experiment uses two similar sources to demonstrate interference: here in water waves. A similar experiment at slightly higher frequency. In this clip, the frequency is about 15% higher. This time the lower view is clear, but the upper view is less clear. By the way: for waves of this size, both the surface tension of water and gravity contribute to the restoring force, so the wave speed is not constant, but is a complicated function of the wavelength, so wavelength and period are not proportional.

Young's experiment with laser light Here helium-neon laser illuminates two parallel slits cut in a metal foil. The slit separation is 0.25 mm and their width is 0.08 mm.   At left, we look towards the screen from above a laser (in the clamp). In front of the laser is a metal foil with two parallel slits cut in it. Top middle pictures show it illuminated from in front with white light and from behind with laser light. On the screen in the image at left, we can see the interference pattern. This is shown in close-up at bottom right. We can see an analogy with the water experiment above: on the axis of symmetry, we see a bright spot, where light from the two sources interferes constructively. A little to the left, a node (black in the pattern). Then an antinode (bright red) where the distance from the two slits must differ by one wavelength. Let's look at the geometry in another diagram.

Young's experiment: the geometry The schematic below shows (not to scale) the relative arrangement of laser, slits and screen (seen from above). The photograph shows the interference pattern. Next to that is a plot of the calculated intensity – we'll return to this when we do interference and diffraction in a later chapter. The central maximum lies on the axis of symmetry: this point on the screen is equidistant from the two slits, so the two waves arrive in phase. On either side we see the first order maxima: also bright regions of the image, where the distances from the two slits differ by one wavelength, which again gives constructive interference. Between them lies a dark patch, where the distances differ by one half wavelength, which produces destructive interference.   A schematic of Young's experiment. It's interesting to note that the photographed pattern doesn't 'look like' the intensity graph plotted beside it. Part of the reason is the nonlinear response of the eye to light intensity: the eye has a dynamic range of about 90 dB, and this cannot be achieved with a linear response. The camera also has a nonlinear response and has probably saturated.

Comparison: Young's experiment with water waves and with light The next illustration is a montage of the two different Young's experiments: consider a horizontal lineabout halfway between the pencils and the bottom of the image and compare it with the image from the laser experiment shown below. (The mapping is not exact, because the optical experiment has a small angle, so sin θ ~ θ, while the water wave pattern occurs over a large angle – in fact, we can observe it over 360°.)   A montage to show the similarities of the interference patters for two similar water wave sources (above) and two similar light wave sources (below). So both water waves and light exhibit interference – a property of waves. But does this explain how light casts shadows? Go to this page about Shadows, particles and waves. This link will return you to the multimedia tutorial The Nature of Light.