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Thermodynamics

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Standard Waves in Light

Similar to stationary waves in sound, due to interference of incident and reflected sound waves in strings and air columns, stationary waves are produced in light. This gives another evidence of the wave nature of light. This phenomenon has been used in colour photography. When a light wave is travelling in the direction of the x-axis and is reflected by a denser medium, a phase change of π occurs between the incident and the reflected light. For the incident wave,

For the reflected wave

The resultant displacement y is given by

Here 2a sin 2πx/λ represents the amplitude of the resultant wave.

For, x = 0, λ/2 , ………. , nλ/2

The amplitude is zero. These are the positions of nodes.

The amplitude is maximum. These are the positions of antinodes. These waves showing alternate formation of nodes and antinodes due to the interference of the incident and the reflected waves are called stationary waves in light.

Wiener experiment: Wiener in 1890 demonstrated the stationary waves formed due to interference of the incident and the reflected light.

In his experiment, Wiener took a highly polished mirror as a reflecting surface. An extremely thin film of photographic emulsion was inclined at a small angle to the reflecting surface. When the film was exposed to light and developed, the film was found to be crossed by a series of dark lines at the positions shown in fig. These are the positions of the antinodal planes. However, no blackening was observed at the nodal planes. However, no blackening was observed at the nodal planes. This experiment shows clearly the formation of stationary waves of light due to interference of the incident and reflected light. In his experiment, Wiener also showed that it is the electric vector (E) perpendicular to the plane of incidence that is responsible for the production of these waves. Moreover, the light must be plane polarized.

In 1932, Fry and Ives repeated Wiener’s experiment using the photoelectric effect in a thin metal film on a wedge of quartz in place of the photographic emulsion.