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.docThe original corpuscular theory was abandoned in the middle of the nineteenth century in favour of the wave theory of light first put forward in 1678 and developed by Young at the end of the 18th century. Young developed an experimental procedure for measuring the wavelength of light. In his experiment Young showed diffraction and interference and that was a good evidence for the wave theory of light.
Young's double-slit experiment provides a quantitative measure of the wavelength and frequency of light waves. Indirectly it enables us to explain why we do not usually observe diffraction of light as it passes through windows, doors, etc. The wavelength of light is too small to show observable diffraction under ordinary conditions.
In Young's double-slit experiment sunlight was allowed to pass through a monochromatic green filter. Green light strikes screen B, containing a small slit S equidistant from slits Sj and S2 in another screen C. Light from S passes through Sj and S2 and finally falls on screen D where it produces bright spots or bands separated by dark areas. The distance between the second screen with slits Sj^ and S2 and the third screen on which the interference pattern appears is 2 m. The slit S in the first screen provides a point source. The purpose of slits Sj and S2 is to provide point sources of light which will interfere and produce bright spots on the third screen. Replacing the green filter with a monochromatic red filter we find that the distance between the bright spots on the third screen increases. It means that the wavelength of red light is longer than that of green. When a violet filter is used in this experiment the distance between the bright spots is less than it is for green light. This means that the wavelength of violet light is less than the wavelength, of green light. Later research has shown, however, that light phenomena must be interpreted in terms of photons and waves, so as the two descriptions are merely two different ways of viewing one and the same reality. Thus, for example, the wave model of the electron is complementary to the particle model.