Young knew that sound was a wave phenomenon, and, hence, that if two sound waves of equal intensity, but out of phase, reach the ear then they cancel one another out, and no sound is heard. This phenomenon is called interference. Young reasoned that if light were actually a wave phenomenon, as he suspected, then a similar interference effect should occur for light. This line of reasoning lead Young to perform an experiment which is nowadays referred to as Young's double-slit experiment.
In Young's experiment, two very narrow parallel slits, separated by a distance , are cut into a thin sheet of metal. Monochromatic light, from a distant light-source, passes through the slits and eventually hits a screen a comparatively large distance from the slits. The experimental setup is sketched in Fig. 86.
According to Huygens' principle, each slit radiates spherical light waves. The light waves emanating from each slit are superposed on the screen. If the waves are out of phase then destructive interference occurs, resulting in a dark patch on the screen. On the other hand, if the waves are completely in phase then constructive interference occurs, resulting in a light patch on the screen.
The point on the screen which lies exactly opposite to the centre point of the two slits, as shown in Fig. 87, is obviously associated with a bright patch. This follows because the path-lengths from each slit to this point are the same. The waves emanating from each slit are initially in phase, since all points on the incident wave-front are in phase (i.e., the wave-front is straight and parallel to the metal sheet). The waves are still in phase at point since they have traveled equal distances in order to reach that point.
From the above discussion, the general condition for constructive interference
on the screen
is simply that the difference in path-length between
the two waves be an integer number of wavelengths. In other words,
Likewise, the general condition for destructive interference
on the screen is that
the difference in path-length between the two waves be a half-integer
number of wavelengths. In other words,
Usually, we expect the wavelength of the incident light
to be much less than the
perpendicular distance to the screen. Thus,
It is clear that the interference pattern on the screen consists of
alternating light and dark bands, running parallel to the slits. The
distances of the centers of the various light bands from the point
are given by
It is interesting to note that when Young first presented his findings to the Royal Society of London he was ridiculed. His work only achieved widespread acceptance when it was confirmed, and greatly extended, by the French physicists Augustin Fresnel and Francois Argo in the 1820s. The particle theory of light was dealt its final death-blow in 1849 when the French physicists Fizeau and Foucault independently demonstrated that light propagates more slowly though water than though air. Recall (from Sect. 14.1), that the particle theory of light can only account for the law of refraction on the assumption that light propagates faster through dense media, such as water, than through rarefied media, such as air.