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interference of monochromatic light

Image from WikiPedia.

For ( φ1 - φ2 )/2 = 0 , π , 2π , ... the cosine part is maximum (brightness), whereas, for ( φ1 - φ2 )/2 = π/2 , 3π/2 , 5π/2 , ... the cosine part is minimum (darkness). This effect can be made visible by shining monchromatic light through two narrow slits. At a screen one observes bright and dark places. The next two videos demonstrate this.

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