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9.2.2 The significance of Young’s experiment

Thomas Young was the first person to demonstrate the wave like nature of light (1803). He showed that when light was passed through two narrow slits that an interference pattern was produced. Since interference is a wave behaviour it disproved the corpuscular theory of light.

Young’s fellow scientists were reluctant to accept his findings because they were still considering light in terms of a longitudinal wave propagated through the ‘ether’ which had evaded all attempts at detection. Also the phenomenon of double refraction could not be explained by a longitudinal wave.

Double refraction is exhibited by calcite crystals called Iceland Spar. Two refracted rays are produced by just one incident ray. One of the refracted rays was found to obey all the laws of refraction ; the other ray did not and was termed the extraordinary ray. The explanation of the behaviour of the extraordinary ray relies on the polarisation direction of the incident ray relative to the orientation of the molecules in the crystal. Since Young and his contemporaries were thinking in terms of longitudinal waves, which cannot be polarised, they could not explain their observations. It took 15 years for Young to realise that a transverse wave could explain the effect and in 1819 Augustin Fresnel devised a mathematical explanation of double refraction based on transverse waves.

Explanation for fringes in general terms

Interference occurs when light from two coherent sources cross. Coherent sources produce waves which have the same frequency, similar amplitudes and a phase difference that does not alter with time. The wavelength of light must be very small otherwise diffraction effects would be much more noticeable than they are. Therefore following experiments with a ripple tank it is clear that the two sources must be very close together and the screen or viewing arrangement must be as far away as possible.

The single slit transmits light from the lamp which then falls on the double slit. Each of the double slits transmits a diffracted beam toward the screen so that there is an area where the diffracted beams overlap. Interference fringes are seen in the area where the beams overlap.

Dark fringes are seen in the positions where light waves from one slit arrive 180° out of phase with waves from the other slit. Bright fringes are seen in positions where waves from one slit arrive in phase with waves from the other slit.

Different colours of light give equally spaced bright and dark fringes parallel to the slits. The spacing of the fringes does depend on the colour (wavelength) of the light. Red light gives a greater fringe spacing than blue light.